Physical unclonable function using divided threshold distributions in non-volatile memory

ABSTRACT

A system and method for utilizing a security key stored in non-volatile memory, and for generating a PUF-based data set on an integrated circuit including non-volatile memory cells, such as flash memory cells, are described. The method includes storing a security key in a particular block in a plurality of blocks of the non-volatile memory array; utilizing, in a security logic circuit coupled to the non-volatile memory array, the security key stored in the particular block in a protocol to enable access via a port by external devices or communication networks to data stored in blocks in the plurality of blocks; and enabling read-only access to the particular block by the security logic for use in the protocol, and preventing access to the particular block via the port.

PRIORITY APPLICATIONS

Benefit of U.S. provisional applications is hereby claimed, including:

-   -   U.S. Provisional Application No. 62/370,736, filed Aug. 4, 2016,        entitled NVM-based Physically Unclonable Function;    -   U.S. Provisional Application No. 62/423,753, filed Nov. 17,        2016, entitled NVM-based Physically Unclonable Function;    -   U.S. Provisional Application No. 62/430,196, filed Dec. 5, 2016,        entitled Non-Volatile Memory Based Physical Unclonable Function;    -   U.S. Provisional Application No. 62/431,835, filed Dec. 9, 2016,        entitled Flash-based Physically Unclonable Function;    -   U.S. Provisional Application No. 62/435,092, filed Dec. 16,        2016, entitled Stable Physically Unclonable Function; and    -   U.S. Provisional Application No. 62/435,337, filed Dec. 16,        2016, entitled Non-Volatile Memory Based Physical Unclonable        Function.

BACKGROUND Field of the Invention

The present invention relates to integrated circuits which include flashmemory arrays or other non-volatile memory arrays, with securityfeatures that utilize a unique key or unique identification code.

Description of Related Art

Integrated circuit memory devices comprising non-volatile memory, suchas flash memory, are being developed with very high capacities. Sometechnologies are considered likely to enable terabit scale arrays onintegrated circuits. Also, memory devices are being deployed in socalled “internet of things IoT” devices, interconnected by networks thatoperate with, for example, internet protocol communicationstechnologies. A concern for IoT devices and other devices that storedata, is data security. Thus, security protocols requiring encryptionwith unique keys, authentication with unique IDs and challenge/responsetechnologies are being deployed.

Security protocols require key management technologies to generate,update, store and protect the unique keys and IDs utilized.

A physical unclonable function (PUF) is a process usable to create aunique, random key for a physical entity such as an integrated circuit.Use of a PUF is a solution for generating keys used for chip IDssupporting a hardware intrinsic security (HIS) technology. PUFs havebeen used for key creation in applications with high securityrequirements, such as mobile and embedded devices. An example PUF is aring-oscillator PUF, that uses the manufacturing variability intrinsicto circuit propagation delay of gates. Another example PUF is an SRAMPUF, where threshold voltage differences in the transistors result inthe SRAM powering up in either a logic “0” or logic “1”. See “PhysicalUnclonable Functions and Applications: A Tutorial” by Charles Herder etal., Pages 1126-1141, Proceedings of the IEEE| Vol. 102, No. 8, August2014.

A PUF that uses the physical property of resistive random access memoryhas been proposed. See “A ReRAM-based Physically Unclonable Functionwith Bit Error Rate<0.5% after 10 years at 125° C. for 40 nm embeddedapplication” by Yoshimoto et al., Pages 198-199, 2016 Symposium on VLSITechnology Digest of Technical Papers. The application presented in thepaper proposes an improvement on the conventional ID-generating methodof ReRAM-PUF which increases bit error rates due to aging degradation.However, in this ReRAM-based PUF, the data created can still becorrupted by drift in the resistance of the memory cells, which can makea bit error rate unacceptable when accessing or using the stored key.Such resistance drift can be more pronounced at high temperatureencountered in some applications of integrated circuits, such as inautomotive applications.

When using data sets generated using PUF circuits, prior arttechnologies have relied upon error correcting codes to improvereliability because of the problems with high bit error rates. See, forexample, Lee et al., U.S. Patent Application Publication No.2016/0156476, “Physically Unclonable Function Circuits And Methods OfPerforming Key Enrollment In Physically Unclonable Function Circuits,”published Jun. 2, 2016.

It is desirable to provide technology for integrated circuits includingnon-volatile memory that support the use of PUF generated keys and otherunique keys.

SUMMARY

A physical unclonable function is described using divided thresholddistributions in non-volatile memory, such as flash memory. Also,devices and systems are described which can improve the flexibility andreliability of security circuitry that utilizes physical unclonablefunctions, and random keys in general. The devices and systems describedherein are suitable for use in internet of things devices, and can beimplemented in a wide variety of environments.

A device as described, for example, which is implemented in a singlepackaged integrated circuit or multichip module that comprises anon-volatile memory array including a plurality of blocks of memorycells with a security key stored in a particular block of the pluralityof blocks. A port for external communication of data from the array isincluded. Security logic is coupled to the memory array which utilizesthe security key in a protocol to enable access to data, or to decryptor encrypt data, stored in the blocks in the plurality of blocks. Accesscontrol circuits are coupled to the array and include logic to enableread-only access to the particular block storing the security key by thesecurity logic for use in protocol, and to prevent access to theparticular block by external communications networks or devices via theport. The access control circuits can have a first state in which accessto the particular block via the port to write the security key isenabled, and a second state in which access to the particular block isdisabled for read or write via the port, but access to the particularblock is enabled for read by the security logic during execution of thesecurity protocol with the host or other external device. The packagedintegrated circuit or multichip module can include logic to execute afunction, including a physical unclonable function such as examplefunctions described herein that rely on charge-trapping non-volatilememory cells as the physical circuits, using a set of memory cells inthe memory array to produce the security key. In embodiments describedherein, a plurality of security keys can be stored on the integratedcircuit in the particular block, or in different blocks. Also, thesecurity logic can be configured to use a particular security key in theplurality of security keys one time, or a limited number of times, toenable access to data stored in blocks in the plurality of blocks. Insome embodiments, the security key stored in the particular block can bea large key, including for example thousands or millions of bits.

A method is provided for generating a data set usable as a uniqueidentifier or key, on an integrated circuit using charge trappingnon-volatile memory cells including floating gate and dielectric chargetrapping technologies, and in some embodiments using other types ofnon-volatile memory cells. The method can include a physical unclonablefunction which results in establishing variant thresholds, such asthreshold voltages, within memory cells in the set. The method can beused to produce a stable, PUF-based data sets, usable with zero or verylow bit error rates.

An apparatus as described comprises a set of charge trapping memorycells, such as flash memory cells. Circuitry, which is on the integratedcircuit, on a processor system having access to the integrated circuit,or includes parts on both, is included, having access to the set ofcharge trapping memory cells used to provide a data set using the set ofcharge trapping memory cells. The data set provided is a function ofvariant threshold voltages of different members of the set of chargetrapping memory cells as a result of a common process that inducescharge trapping in charge storage structures in the memory cells.According to one aspect of the technology, the set of charge trappingmemory cells has an order which can be represented by the addresses, andthe variant threshold voltages have a starting distribution. The dataset according to this aspect of the technology is a function ofpositions in the order of a subset of the set of charge trapping memorycells having threshold voltages in a particular part of the startingdistribution.

A method for generating a data set on an integrated circuit includingprogrammable memory cells, such as flash memory cells, is described. Themethod includes exposing a set of programmable memory cells havingaddresses on the integrated circuit to a common process inducing variantthresholds and members of the set within a starting distribution ofthresholds. The method includes finding a first subset of the set ofprogrammable memory cells having thresholds in a first part of thedistribution, and a second subset of the set of programmable memorycells having thresholds in a second part of the starting distribution.The method includes generating a data set using addresses of at leastone of the first and second subsets.

In one embodiment, the data set is generated using the addresses toselect memory cells in one of the first and second subsets, and apply abiasing operation to the selected memory cells to establish a sensingmargin between the first and second subsets of memory cells. The sensingmargin can be established in one embodiment, by addressing the memorycells in a selected one of the subsets, and applying a biasing operationto the addressed memory cells which changes their threshold to athreshold distribution outside of the starting distribution. The dataset can be generated thereafter, by reading memory cells in the set ofprogrammable memory cells using a read voltage that is within thesensing margin.

In another embodiment, the data set is generated by combining theaddresses of memory cells in at least one of the first and secondsubsets as a function of membership in the subsets, and as a function oftheir addresses. One technique for combining the addresses can compriseconcatenating the addresses of one of, or each of, the subsets in order.The data set comprising the combined addresses can be stored in a memoryon the integrated circuit which is different from the set ofprogrammable memory cells.

In another technique described herein, the data set is generated byfinding a first dividing line and a second dividing line different thanthe first dividing line, in the starting distribution; identifying afirst subset of the set of the programmable memory cells havingthresholds below the first dividing line in a first part of the startingdistribution, and a second subset of the set of the programmable memorycells having thresholds above the second dividing line in a second partof the starting distribution; and generating a data set using addressesof at least one of the first and second subsets.

One approach for finding the dividing lines includes determining athreshold voltage in the starting distribution at which a ratio of acount of memory cells having thresholds below the threshold voltage to acount of the memory cells having thresholds above the threshold voltageis within a target range of ratios, and setting the first dividing lineby subtracting a first constant from the threshold voltage, and settingthe second dividing line by adding a second constant to the thresholdvoltage. Another approach for finding the dividing lines includesiteratively reading the data values in the set of programmable memorycells using a moving first read voltage, and counting memory cells inthe set having thresholds below the first read voltage, and setting thedividing line using the first read voltage at which the count is withina first target range of counts; and iteratively reading the data valuesin the set of programmable memory cells using a moving second readvoltage, and counting memory cells in the set having thresholds abovethe second read voltage, and setting the second dividing line using thesecond read voltage at which the count is within a second target rangeof counts.

A method of manufacturing an integrated circuit in accordance with themethod for generating a data set provided herein is also described.

An apparatus is described that comprises a set of programmable memorycells on an integrated circuit, and logic to generate a data set byprocesses described herein.

Other aspects and advantages of the present invention can be seen onreview of the drawings, the detailed description and the claims, whichfollow.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified block diagram of an apparatus comprising aplurality of flash memory cells, and a controller for executing a PUF toprovide a data set using the plurality of flash memory cells.

FIG. 2 illustrates another example of apparatus comprising a pluralityof programmable memory cells, and a controller for executing a PUF toprovide a data set using the plurality of programmable memory cells.

FIG. 3 illustrates blocks of programmable memory cells in a Flash memoryon an integrated circuit, including a PUF block.

FIGS. 4A-4E illustrate an example of generating a data set by findingfirst and second subsets of a set of programmable memory cells andestablishing a stable data set based on the identified first and secondsubsets.

FIGS. 5A-5E illustrate another instance of generating a data set asdiscussed with reference to FIGS. 4A-4E, showing variations in the dataset even though the same process is applied.

FIG. 6 shows an example flowchart for generating a stable data set on anintegrated circuit including programmable memory cells, as describedwith reference to FIGS. 4A-4E and 5A-5E.

FIGS. 7A-7E illustrate an example of generating a data set by findingfirst, second and third subsets of a set of programmable memory cellsand building an address map for the data set.

FIG. 8 shows an example flowchart for generating a data set on anintegrated circuit including programmable memory cells, as describedwith reference to FIGS. 7A-7E.

FIG. 9 shows another example flowchart for generating a data set on anintegrated circuit including programmable memory cells, as describedwith reference to FIGS. 7A-7E.

FIGS. 10A-10C illustrate setting thresholds of the programmable memorycells in the set to the starting distribution.

FIGS. 10D-10F illustrate example flash memory cell technologies usablefor generating data sets as described herein.

FIG. 11 shows an example flowchart for applying an initializing biasingoperation setting thresholds of the programmable memory cells in the setto the starting distribution.

FIGS. 12A-12C illustrate a process which can be used for generating adata set in reliance on the variance in threshold voltages in chargetrapping memory cells.

FIG. 13 is an example flowchart for generating a stable data set on anintegrated circuit including programmable, charge trapping memory cellsin a manner explained with reference to FIGS. 12A-12C.

FIGS. 14A-14C illustrate an alternative process which can be used forgenerating a data set in reliance on the variance in threshold voltagesin charge trapping memory cells.

FIG. 15 is an example flowchart 1500 for generating a stable data set onan integrated circuit including programmable, charge trapping memorycells in a manner explained with reference to FIGS. 14A-14C.

FIG. 16 is a simplified block diagram of an integrated circuit includinga flash memory array and a controller for utilizing the memory array toprovide a data set.

FIG. 17 is a system diagram showing a packaged integrated circuit ormultichip module including physical unclonable function circuitry andnon-volatile memory, coupled with an enrollment system.

FIG. 18 illustrates an alternative configuration of non-volatile memoryusable in a system like that of FIG. 17.

FIG. 19 illustrates another alternative configuration of non-volatilememory usable in a system like that of FIG. 17.

FIG. 20 illustrates a data structure usable to store security ID andaddress map which are produced according to some embodiments of physicalunclonable functions as described herein, and can be stored in a systemsuch as that of FIG. 17.

FIG. 21 is a simplified system diagram including a packaged integratedcircuit or multichip module and host utilizing a physical unclonablefunction circuit with non-volatile memory.

FIGS. 22-24 are simplified flowcharts illustrating operation of a systemlike that of FIG. 21 in various embodiments.

FIGS. 25-28 provide simplified illustrations of various alternativeconfigurations of physical unclonable circuitry combined withnon-volatile memory as described herein.

DETAILED DESCRIPTION

A detailed description of embodiments of the present technology isprovided with reference to the Figures. It is to be understood thatthere is no intention to limit the technology to the specificallydisclosed structural embodiments and methods but that the technology maybe practiced using other features, elements, methods and embodiments.Preferred embodiments are described to illustrate the presenttechnology, not to limit its scope, which is defined by the claims.Those of ordinary skill in the art will recognize a variety ofequivalent variations on the description that follows. Like elements invarious embodiments are commonly referred to with like referencenumerals.

FIG. 1 is a simplified block diagram of an apparatus comprising aplurality of programmable memory cells, and a controller for executing aprocess to provide a data set using the plurality of programmable memorycells. In this example, the apparatus comprises an integrated circuit100 having a memory formed using programmable memory cells. The memory130 can used by a PUF to provide a unique data set. Another embodiment,in which the mission function circuits 110 are a flash memory arraycomprising a plurality of blocks of memory cells is described below withreference to FIG. 17. Other embodiments are described herein as well.

The integrated circuit 100 includes mission function circuits 110, whichcan comprise special purpose logic sometimes referred to asapplication-specific integrated circuit logic, data processor resourcessuch as used in microprocessors and digital signal processors,large-scale memory such as flash memory, SRAM memory, DRAM memory,programmable resistance memory and combinations of various types ofcircuits known as system-on-a-chip SOC configurations or applicationspecific integrated circuits ASICs. The integrated circuit 100 includesan input/output interface 120, which can comprise wireless or wiredports providing access to other devices or networks. In this simplifiedillustration, an access control block 115 is disposed between theinput/output interface 120, and the mission function circuits 110. Theaccess control block 115 is coupled by bus 116 to the input/outputinterface 120, and by bus 111 to the mission function circuits 110. Anaccess control protocol is executed by the access control block 115 toenable or disable communications between the mission function circuits110 and the input/output interface 120, to provide encryption ordecryption of data traversing the input/output interface 120, to provideother services in support of the security logic or to providecombinations of the same.

In support of the access control block 115, security logic 125 isdisposed on the chip in this example. Security logic 125 is coupled to aset of flash memory cells which can be part of flash memory array 130. APUF then provides or is used to provide a unique data set, which uniquedata set is accessible on a bus 131 by the security logic 125 through aPUF program controller 140 on bus 141, and utilized by the securitylogic in communications across line 122 with the access control block115.

In this example of the apparatus, the PUF program controller 140,implemented for example as a state machine on the integrated circuitwith the flash memory array 130, provides signals to control theapplication of bias arrangement supply voltages to carry out theprocedures to generate the data set, and other operations involved inaccessing the array 130 and for reading the data set provided using thememory array 130. Circuitry, which is on the integrated circuit such asbit lines, word lines, drivers for the same and so on, provides accessto the set of charge trapping memory cells used to provide a data setusing the set of charge trapping memory cells.

A PUF program controller 140 on the integrated circuit includes logic toperform some or all of operations used to generate the data set. In oneembodiment, the PUF program controller 140 on the integrated circuitincludes the logic necessary to perform the biasing operations, and canexecute the logic in response to a set up command from an externalsource, without control from an off-chip system.

The controller can be implemented using special-purpose logic circuitryincluding a state machine as known in the art. In alternativeembodiments, the controller comprises a general-purpose processor, whichcan be implemented on the same integrated circuit, which executes acomputer program to control the operations of the device. In yet otherembodiments, a combination of special-purpose logic circuitry and ageneral-purpose processor can be utilized for implementation of thecontroller.

In some embodiments an external processor system can include circuitryfor providing access to the integrated circuit and logic used forgeneration of the data set. The external processor system can includecircuitry such as wafer probe circuits, control buses, voltage sourcesand the like used to provide the data set in combination with thecircuitry on the integrated circuit. Logic circuits and biasingcircuitry having access to the set of memory cells used to control theprocedures can include parts on both the external processor system andthe integrated circuit.

The examples described herein utilize charge trapping memory cells suchas utilized in some kinds of flash memory. The charge storage structuresin charge trapping memory cells can include polysilicon or otherconductive or semi-conductive floating gate structures, and can includemultilayer dielectric charge trapping structures known from flash memorytechnologies as ONO (oxide-nitride-oxide), ONONO(oxide-nitride-oxide-nitride-oxide), SONOS(silicon-oxide-nitride-oxide-silicon), BE-SONOS (bandgap engineeredsilicon-oxide-nitride-oxide-silicon), TANOS (tantalum nitride, aluminumoxide, silicon nitride, silicon oxide, silicon), and MA BE-SONOS(metal-high-k bandgap-engineered silicon-oxide-nitride-oxide-silicon).

In other embodiments, the programmable memory cells used in the PUFmemory cells to provide the data set can include programmable resistancememory cells or other types of memory cells. The programmable resistancememory cells used to provide the data set can include a programmableelement having a programmable resistance readable with reference tothreshold resistances. The programmable resistance element can comprisefor example, a metal oxide or a phase change material.

FIG. 2 illustrates another example of an apparatus comprising a set ofprogrammable memory cells on an integrated circuit, and logic togenerate a data set using the set of programmable memory cells. In thisexample, the apparatus comprises a processor system 410 used forexecuting or causing execution of a process to generate a data set asdiscussed herein, on an integrated circuit 440. The integrated circuit440 is connected to the system 410 during manufacturing beforepackaging, such as in wafer form in some embodiments. In otherembodiments, the system 410 is connected to the integrated circuit in apackaged form.

An example system used for executing a process to generate a PUF-baseddata set on an integrated circuit can include programmed processexecuted in a manufacturing line using equipment used for testing, orusing equipment like that used for testing, which includes circuitry foraccessing the integrated circuit such as wafer probe circuits, voltagesources, and the like. For example, a manufacturing line may havemultiple device testers, multiple device probers, multiple devicehandlers, and multiple interface test adapters configured to connect tothe integrated circuits which can be configured to control execution ofthe procedures described herein. In an alternative, a system may beconfigured to interact with packaged integrated circuits, and may bedeployed away from the manufacturing line for the integrated circuit,such as at an assembly installation for an original equipmentmanufacturer utilizing the integrated circuits.

As shown in FIG. 2, an example system 410 includes PUF logic and driver420, and a device handler/prober 430. An integrated circuit 440 to besubjected to the PUF logic and driver 420 is coupled to the devicehandler/prober 430. The integrated circuit 440 includes a securitycircuit 450. A flash memory array 460 in the security circuit 450 inthis example is utilized for generation of the data set using the PUF.

An example integrated circuit in the system 410 may be an integratedcircuit 100, as described with reference to FIG. 1. During manufacturingof the integrated circuit 100, the system 410 performs the actionsidentified herein to produce the data set and can save a copy of thedata set, or data derived from the data set, as a shared secret betweenthe integrated circuit and a processor (e.g. 410) in the factory. Inalternative embodiments, in the field after manufacturing of anintegrated circuit, a user can generate a data set in the memory array460 on the integrated circuit, for example using the system 410, so thedata set can be saved as a shared secret between the integrated circuitand a processor (e.g. 410) in the field, rather than in the factory.

FIG. 3 illustrates a large-scale flash memory array 470 which can be themission function circuits or part of the mission function circuits on anintegrated circuit, with which the security circuit as described hereinis utilized. A flash memory array 470 can include blocks of flashprogrammable memory cells (e.g. Memory Blocks 0, 1, . . . N), a PUFblock (471), a boot block 472 and a parameter block 473. In addition, aflash memory array can include protection logic 474 for controllingaccess to the various memory blocks in the array, which includes memoryfor protection bits. The PUF block 471 can be a particular block in thememory array reserved for, and in some embodiments, specially configuredfor the purpose of storing security keys.

In embodiments in which the integrated circuit includes a flash memoryarray as shown in FIG. 3, the flash memory array 460 in the securitycircuit 150 of the integrated circuit can comprise a block, such as thePUF block 471, in the large-scale flash memory array 470. In otherembodiments, the flash memory array 460 in the security circuit 450 isseparate from the flash memory array 470, and may comprise memory cellshaving a different structure and an array having a differentarchitecture than the large-scale memory array 460. Another embodimentincluding a large scale flash memory is described with reference to FIG.17 below.

The flash memory array 470 can comprise NOR flash, NAND flash, or othertypes of flash architectures. As a PUF algorithm as described herein isexecuted over a set of memory cells, the PUF block 471 can compriseenough memory cells to encompass one set, or many sets of suitable sizefor use by the security circuit 450 for the creation of a PUF data setor many PUF data sets. The read, program and erase logic in theperipheral circuitry 475 associated with the flash memory array 470 canbe utilized by the PUF logic and driver 420, or by a state machine onthe integrated circuit as discussed above, or by a combination of thedriver 420 and a state machine on the integrated circuit, to applybiasing arrangements to change the threshold voltages of memory cells inthe PUF block 471 according to the PUF procedures described herein.

The PUF block can be supported by the protection logic 474 to preventaccidental or unauthorized access to the PUF-based data set or to thememory cells storing the data set. The boot block can include a writelock-out feature to guarantee data integrity for the integrated circuitincluding the memory array. The boot block can store the code necessaryto initialize the integrated circuit and invoke a recovery routine ifthe code is lost. The boot block can store the code necessary to programand erase the flash memory array in the integrated circuit. Theparameter block can store parameter data. The protection bits arecoupled to the memory blocks and the PUF ID block for their protectionfrom accidental or unauthorized modification. One example of protectionof blocks of memory from modification including using protection codesis shown in Hung et al., U.S. Patent Application Publication No. US2015-0242158, entitled “Nonvolatile Memory Data Protection UsingNonvolatile Protection Codes and Volatile Protection Codes,” publishedAug. 27, 2015, which is incorporated by reference as if fully set forthherein.

Reference to FIGS. 4A-4E is made to illustrate a process which can beused for generating a data set in reliance on the variance in thresholdvoltages in charge trapping memory cells which have been subject to acommon process, such as a manufacturing sequence, or a common biasingarrangement, which result in charge tunneling into or out of chargestorage structures in the memory cells changing the amounts of chargestored in the charge storage structures. The common process whichresults in establishing a starting distribution can be “unclonable” inthe sense that the threshold voltages that result from this commonprocess differ from one set of charge trapping memory cells to another,and from one integrated circuit to another based on variations inprocess, voltage and temperature in the individual charge trapping cellsin the sets. For this reason, even knowing the common process, onecannot predict the variations in threshold voltage, and therefore cannotpredict a resulting data set generated as a function of thosevariations.

FIG. 4A is a graph of threshold voltage versus cell count, illustratinga starting threshold distribution 500 for memory cells in a set ofprogrammable memory cells, which is established at the beginning of thePUF process. For the purposes of example, points are indicated withinthe distribution that represent the threshold voltages of memory cellsat addresses Addr=0, 1, 2, 3, 4 and 5. As can be seen, the thresholdvoltages of particular cells are not related to the addresses of thememory cells.

The term “address” is used here to represent a logical signal that canbe used to select a memory cell according to a physical order of thememory cells. In memory technologies, addresses are decoded to generatelogical signals to control biasing circuitry used to access the memorycells. In some implementations, the “address” may be a logical signalthat does not require decoding. In some implementations, the “address”of a cell may be a bit in a mask or mapping table, such as shown in FIG.20 below. The addresses can be combined by forming a mask, and thecombined addresses can be stored in the form of a mask in which eachentry in the mask enables or blocks a memory cell at the correspondingaddress.

The starting distribution 500 can occur due to charge trapping naturallyon completion of manufacturing as a result of etching or depositionprocesses, such as processes involving exposure of the integratedcircuit to plasma or ions, for formation of patterned metal layers abovethe memory cells used in manufacturing. In alternative embodiments, thestarting distribution 500 can be established using for example biasingoperation controlled by a controller on the integrated circuit, such asan erase operation or the like as described below. In one example, thestarting distribution is established for all members of the set of theprogrammable memory cells using a page erase operation or a block eraseoperation, where a block includes multiple pages of programmable memorycells. See U.S. Patent Application Publication No. 2016/0284413 A1titled “Page Erase in Flash Memory,” published Sep. 29, 2016. Theprocesses that result in establishing the starting distribution areperformed without distinguishing among the memory cells in the set byaddresses. The processes that result in establishing the startingdistribution can be a physical unclonable function, such that thestarting distribution is unique, for each set of programmable memorycells subjected to the processes.

In this illustration, the starting distribution 500 has an upperthreshold level as indicated in the figure, indicating a threshold levelat which the probability of a memory cell in the set having a higherthreshold is very low. This upper threshold level might be set forexample in an algorithm used to establish the starting distribution 500as an erase verify level for example.

FIG. 4B illustrates a next stage in the PUF, in which memory cellshaving thresholds below a dividing line are identified as members of asubset of memory cells having threshold voltages in a first part 510 ofthe starting distribution. Also, memory cells having thresholds abovethe dividing line are identified as members of a subset of memory cellshaving threshold voltages in a second part 520 of the startingdistribution. Thus, the set of programmable memory cells has a firstsubset having thresholds in the first part of the starting distribution(e.g. Addr=0, 3 and 4), and a second subset having thresholds in thesecond part of the starting distribution (e.g. Addr=1, 2 and 5).

The addresses of memory cells in the first and second subsets can bebuilt by applying a scanning operation on the programmable memory cellsusing a read voltage on the dividing line, and recording the addressesof memory cells which return a first logical state as the first subset,and the addresses of the memory cells which return a second logicalstate as the second subset. Recording the addresses can preserveinformation about the location of the memory cells in each of thesubsets used to provide the data set.

In some embodiments, the dividing line can be determined using a findingoperation that generates a count of the programmable memory cells in thefirst subset (having thresholds below the dividing line) and a count ofthe programmable memory cells in the second subset (having thresholdsabove the dividing line). The counts can be compared to produce a ratio.The ratio can be set at a value which ensures that the numbers of zerosand ones in the data set are sufficient to maintain a secure data set.For example, it may be desirable that the ratio of zeros to ones beclose to 1. For a practical embodiment, a target ratio range can be forexample between 2/3 and 3/2, in which case each subset has about 40% to60% of the memory cells in the whole set. The target ratio range can beadjusted according to design specifications of particular integratedcircuits that use the technology as described herein.

The threshold voltages of individual charge trapping cells in the setcan drift over time, so that starting distribution 500 represents adistribution that is stable only for a short time. Thus, relying on thestarting distribution for the purposes of producing a stable data set isimpractical for some types of memory cells where this drift can causethreshold voltages in some cells having thresholds on one side of thedividing line at one point in time to drift to the other side of thedividing line. Thus, techniques are applied to translate the startingdistribution into a stable data set which is not sensitive to this driftin threshold voltages.

One technique for translating the starting distribution into a stabledata set involves the use of the addresses of memory cells in the firstand second subsets. In this technique, the data set based on thesevariant thresholds can comprise a concatenation of the addresses of oneor both of the subsets, or a mask including entries blocking or enablingcells at the addresses. In the example of FIG. 4A, the concatenation ofaddresses of the first subset and the second subset can be can be 0, 3,4, . . . , 1, 2, 5 . . . expressed in binary form in the figure, orvariations of this sequence. Of course, in a particular embodiment,there can be hundreds, thousands or millions of addresses in theconcatenation of addresses. The concatenated addresses can be stored onthe integrated circuit in a memory, such as a protected block of memorydifferent from the set as represented by FIG. 4D. This protected blockof memory can be a non-volatile, stable memory providing the ability todeliver the data set with very low bit error rates, and without usingerror correcting codes in some examples. In alternative embodiments, theaddresses of cells in the first and second subsets can be indicated by amask, such as described below with reference to FIG. 20.

As indicated in the diagram, according to another technique, a stabledata set based on the threshold voltages can be created using the sameset of non-volatile memory cells. In order to accomplish thisembodiment, a programming operation can be executed on memory cells inthe second subset to move their threshold voltages above a first verifylevel such in in a distribution 525 shown in FIG. 4C, which in thisexample can be higher than this upper threshold level of the startingdistribution 500. In other embodiments, the first verify level may beless than the upper threshold level of the starting distribution 500, solong as a sufficient read margin can be produced as discussed below.

After executing the program operation using the first verify level, achanged distribution like that shown in FIG. 4C results. The programoperation changes the threshold voltages of the memory cells in thesecond subset in order to establish a sensing margin 530 between thefirst and second subsets. The sensing margin 530 can be designed to bewide enough to ensure reliability of an operation reading the data setto determine whether a particular memory cell is a member of the firstsubset or the second subset. After the changed distribution shown inFIG. 4C is established, the set of memory cells can be read using a readvoltage V_(R) that is within the read margin 530. The read margin 530can be substantial, so that the likelihood of an error in the readingoperation is very low.

FIG. 4E is a table representing the data set stored in the same set ofmemory cells, showing sequential addresses of the memory cells in theset in the first column and a data value (or key) in the second column,which represents a data set generated by the PUF process. In this table,reading memory cells at addresses 0 through 5 yields data values 1 0 0 11 0. In practical embodiments, the data set can be hundreds, thousandsor millions of bits long.

FIGS. 5A-5E are a sequence of drawings just like FIGS. 4A-4E,representing another instance of executing the same PUF process on a setof charge trapping memory cells. In this instance, the resulting dataset is different, even though the PUF process is the same.

FIG. 5A illustrates a starting threshold distribution 600 of a set ofprogrammable memory cells, including memory cells at addresses Addr=0,1, 2, 3, 4 and 5. The starting distribution has an upper thresholdlevel.

FIG. 5B illustrates a first part 610 and a second part 620 of thestarting distribution. The set of programmable memory cells has a firstsubset having thresholds in the first part of the starting distribution(e.g. Addr=1, 2, 3 and 4), and a second subset having thresholds in thesecond part of the starting distribution (e.g. Addr=0 and 5). The dataset based on these variant thresholds can comprise a concatenation ofthe addresses of some or all of the cells in one or both of the subsets.In the example of FIG. 5A, the concatenation of addresses of some or allof the cells in the first subset and some or all of the cells in thesecond subset can be (1, 2, 3, 4, . . . ); (0, 5 . . . ) expressed indigital form, or variations of this sequence. As mentioned above, theconcatenated addresses can be stored on the integrated circuit in amemory, such as a protected block of memory different from the set asrepresented by FIG. 5D, where the addresses are shown in binary form.This protected block of memory can be a non-volatile, stable memoryproviding the ability to deliver the data set with very low bit errorrates, and without using error correcting codes in some examples.

FIG. 5C illustrates the result after a biasing operation (e.g.programming) is applied to the second subset of programmable memorycells to establish a sensing margin 630 between the first and secondsubsets. For example, the second subset can include the programmablememory cells having thresholds above the dividing line when the ratio iswithin the target ratio range. The biasing operation changes thethresholds of the programmable memory cells in the second subset to athreshold distribution 625. The threshold distribution 625 can be abovethe first verify level.

FIG. 5D shows a data set based on these variant thresholds comprising aconcatenation of the addresses of one or both of the subsets, or on amask including entries for the cells at the addresses. In the example ofFIG. 5B, the concatenation of addresses of the first subset and thesecond subset can be can be (1, 2, 3, 4, . . . ); (0, 5 . . . )expressed in binary form in the figure, or variations of this sequence.Of course, in a particular embodiment, there can be many addresses inthe concatenation of addresses. The concatenated addresses can be storedon the integrated circuit in a memory, such as a protected block ofmemory different from the set as represented by FIG. 5C. This protectedblock of memory can be a non-volatile, stable memory providing theability to deliver the data set with very low bit error rates, andwithout using error correcting codes in some examples.

FIG. 5E is a table showing sequential addresses of memory cells in theset in the first column and a data value (or key) in the second column,which represents a data set generated by the PUF process. In this table,reading memory cells at addresses 0 through 5 yields data values 0 1 1 11 0, which is different than that generated in the instance representedby FIGS. 4A-4E.

Thus, the data set is a function of variant threshold voltages ofdifferent members of the set of charge trapping memory cells as a resultof a common process that induces charge trapping in the set. This resultcan be achieved using different sets of programmable memory cells foreach instance of the PUF process. For some types of memory cells, aplurality of data sets can be generated using the same set ofprogrammable memory cells by creating new starting distributions foreach new data set. Also, for a data set stored in the same memory cellsas the set used for establishing the variant distribution, an old dataset can be replaced by a new data set.

FIG. 6 is a flowchart 200 for generating a stable data set on anintegrated circuit including programmable, charge trapping memory cellsin the manner explained with reference to FIGS. 4A-4E and 5A-5E. In thisexample, the process begins with a set of flash memory cells having astarting distribution of thresholds (201), having been established by acommon PUF process which results in variant threshold voltages asdiscussed above. In this example, the process includes finding memorycells, such as by determining the addresses, within the set of memorycells of a first subset having thresholds above a dividing line, and asecond subset having thresholds below a dividing line (210). Thedividing line can be chosen empirically, and stored as a parameter inthe system controlling execution of the PUF. Alternatively, asillustrated in this example, the dividing line can be adjusted accordingto the characteristics of the starting distribution and the desiredcharacteristics of the data set. In this alternative, the processdetermines a ratio of a count of memory cells in the first subset to thecount of memory cells in the second subset (220). This ratio can be anumber such as 1/1, or fall in an acceptable range 3/2 through 2/3 forexample, chosen according to the intended uses of the data set.

If the ratio is not acceptable (230), then the process adjusts thedividing line (240) and returns to step 210 to identify the first andsecond subsets. If the ratio is acceptable (230), then the process movesto a step of establishing a stable data set based on the identifiedfirst and second subsets of the memory cells. As mentioned above, in onealternative, the process can store the addresses in the first subset insequence, and the addresses in the second subset in sequence, and usethe concatenated addresses as the stable data set.

In the alternative illustrated in FIG. 6, the process to establish astable data set includes applying a biasing operation to the memorycells in the second subset to establish a changed distribution in theset of memory cells, having a sensing margin between the memory cells inthe first subset and the memory cells in the second subset (250). Forcharge trapping memory cells, this biasing operation can comprise aprogram operation such as incremental step pulsed programming ISPP usinga verify level sufficiently above the dividing line of the thresholdvoltages to establish the sensing margin. The biasing operation can beapplied only to the memory cells having addresses in the second subset.In this way, the identification of the memory cells in the first subsetand of the memory cells in the second subset can be determined by a readoperation using a read voltage located within the sensing margin. Theprocess of FIG. 6 includes therefore a step of outputting the data setby reading a sequence of memory cells in the set using a read voltage inthe read margin (260). The output data set can be provided to anexternal system, such as the system controlling execution of the PUF foruse as a shared secret in a security protocol. The data set can bestably stored in the set of memory cells because the sensing marginestablishes a very low likelihood that memory cells initially identifiedas members of the second subset would have their threshold voltagesdrift into the range identified with the first subset.

FIGS. 7A-7E illustrate another technique useable to generate a stabledata set based on a starting distribution of thresholds.

FIG. 7A illustrates a starting threshold distribution 700 of a set ofprogrammable memory cells, including memory cells at addresses Addr=0,1, 2, 3, 4, 5 and 6. The starting distribution can have an upperthreshold level as illustrated above in which it is very unlikely thatany memory cells in the starting distribution will have thresholds.

FIG. 7B illustrates a first part 710, a second part 720, and a thirdpart 730 of the starting distribution. The first part of the startingdistribution includes thresholds less than the upper threshold level andbelow a first dividing line, the second part of the startingdistribution includes thresholds less than the upper threshold level andabove a second dividing line that is greater than the first dividingline, and the third part of the starting distribution includesthresholds between the first and second dividing lines. A process can beincluded for finding one or both of the first and second dividing linesbased on desired characteristics of the data set.

The set of programmable memory cells has a subset having thresholds inthe first part of the starting distribution (e.g. Addr=0 and 3), asubset having thresholds in the second part of the starting distribution(e.g. Addr=2 and 5), and a subset having thresholds in the third part ofthe starting distribution between the first read level and the secondread level.

The threshold voltages of individual charge trapping cells in the setcan drift over time, so that starting distribution 700 represents adistribution that is stable only for a short time. Thus, relying on thestarting distribution for the purposes of producing a stable data set isimpractical for some types of memory cells where this drift can causethreshold voltages in some cells having thresholds on one side of adividing line at one point in time drift to the other side of thedividing line. Thus, techniques are applied to translate the startingdistribution into a stable data set which is not sensitive to this driftin threshold voltages.

As indicated in the diagram, one technique for establishing a stabledata set based on the threshold voltages can use the same set ofnon-volatile memory cells. In order to accomplish this embodiment, aprogramming operation can be executed on memory cells in the subsethaving thresholds between the first and second dividing lines to movetheir threshold voltages above a first verify level, which in thisexample is higher than this upper threshold level of the startingdistribution 500. In other embodiments, the first verify level might beless than the upper threshold level of the starting distribution 500, solong as a sufficient read margin can be produced as discussed below.

After executing the program operation using the first verify level, achanged distribution like that shown in FIG. 7C results. The programoperation changes the threshold voltages of the memory cells in order toestablish a sensing margin 740 between the first and second subsets. Thesensing margin 740 can be designed to be wide enough to ensurereliability of an operation reading the data set using a read voltage inthe sensing margin to determine whether a particular memory cell is amember of the first subset or the second subset. After the changeddistribution shown in FIG. 4C is established, the set of memory cellscan be read using a read voltage V_(R) that is within the sensing margin740. The sensing margin 740 can be substantial, so that the likelihoodof an error in the reading operation is very low.

FIG. 7D is a table representing the data set stored in the same set ofmemory cells, showing sequential addresses of the memory cells in theset in the first column and a data value (or key) in the second column,which represents a data set generated by the PUF process. In this table,reading memory cells at addresses 0 through 6 yields data values 1 X 0 1X 0 X, where “X” is a don't care because it corresponds to the result ofsensing a memory cell in the subset that was subjected to the biasingoperation to establish the sensing margin. In practical embodiments, thedata set can be hundreds or thousands of bits long.

In this example, first and second subsets of the set of the programmablememory cells are used to provide a data set (e.g. “Key Data”). Forexample, a data set of 1010 is provided using programmable memory cellsin the first and second subsets, at addresses Addr=0, 2, 3 and 5, wheredata “1” is provided using programmable memory cells in the first subsetat addresses Addr=0 and 3, and data “0” is provided using programmablememory cells in the second subset at addresses Addr=2 and 5. The dataset does not include the data in a subset of the set of the programmablememory cells used to establish the sensing margin, e.g., at addressesAddr=1, 4 and 6, which have thresholds in the threshold distribution 735outside the starting distribution. The “X” shown indicates the memorycells not used for the data set in this example.

The biasing operation in this example can also establish in someembodiments a second sensing margin 750 between the others of thesubset, which is wide enough to ensure reliability of sensingdistinctions in threshold voltage between the second and third subsetsof the programmable memory cells even under conditions in which PVT(process, voltage, temperature) variations are relatively large. Suchinformation could be used in the generation of the data set.

The address map can be built by applying a scanning operation on theprogrammable memory cells in the set of programmable memory cells torecord addresses of programmable memory cells in the first, second andthird subsets that are used to provide the data set. For example, theaddresses of programmable memory cells in the third subset can be markedwith a skip flag, so the programmable memory cells in the third subsetwill not be read for providing a data set that can be used as a securitykey for an authentication or encryption protocol, or other type ofsecret or unique data value. Alternatively, mask logic can be used asdiscussed with reference to FIG. 20 below.

In response to a challenge, a PUF ID circuit (e.g. 125, FIG. 1; 450,FIG. 2) can provide a security key using a data set provided using theprogrammable memory cells having addresses recorded in the address map,according to the addresses of the first and second subsets of the set ofthe programmable memory cells recorded in the address map, therebyskipping or not using the programmable memory cells having addresses inthe third subset of the set of the programmable memory cells.

As discussed above, in alternative processes, the data set based onthese variant thresholds can comprise a combination, such as aconcatenation, of the addresses of some or all of the cells in one orboth of the subsets. FIG. 7E shows a data set based on these variantthresholds comprising a concatenation of the addresses of one or more ofthe subsets. In the example of FIG. 7E, the concatenation of addressesof a first subset including the memory cells found between the dividinglines, and a second subset including memory cells found below the firstdividing line can be can be (1, 4, 6, . . . ); (0, 3, . . . ) expressedin binary form in the figure, or variations of this sequence. Of course,in a particular embodiment, there can be hundreds or thousands ofaddresses in the concatenation of addresses. The concatenated addressescan be stored on the integrated circuit in a memory, such as a protectedblock of memory different from the set of memory cells having thestarting distribution. This protected block of memory can be anon-volatile, stable memory providing the ability to deliver the dataset with very low bit error rates, and without using error correctingcodes in some examples.

In the examples of FIGS. 4D, 5D and 7E, the data set is a concatenationof addresses one or more of the subsets, such as some or all of thecells in a first subset, some or all of the cells in a second subset,and some or all of the cells in a third subset can be (0, 3, . . . );(2, 5 . . . ); (1, 4, 6, . . . ), or variations of this sequence. Insome embodiments, the addresses of only the memory cells in one of thesubsets, for example the third subset can be used as the data set. Theaddresses can be combined using logical functions other than or inaddition to concatenation, such as a hash function or formation of amask, to form a data set including the combination of addresses.

FIG. 8 is a flowchart 800 for generating a stable data set on anintegrated circuit including programmable, charge trapping memory cellsin a manner explained with reference to FIGS. 7A-7E. In this example,the process begins with a set of flash memory cells having a startingdistribution of thresholds (801), having been established by a commonprocess which results in variant threshold voltages as discussed above.In this example, the process includes determining addresses within theset of memory cells of a first subset of memory cells having thresholdsbelow a first dividing line, a second subset having thresholds above asecond dividing line, and a third subset having thresholds between thedividing lines (810). The dividing lines can be chosen empirically, orotherwise, and stored as a parameter in the system controlling executionof the PUF. Alternatively, as illustrated in this example, the dividinglines can be adjusted according to the characteristics of the startingdistribution, and the desired characteristics of the data set. In thisalternative, the process determines a ratio of a count of memory cellsin the first subset to the count of memory cells in the second subset(820). Then the algorithm determines whether the ratio matches anexpected range (830). If at step 830, is determined that the ratio isnot okay, then the dividing lines are adjusted (835), and the processreturns to step 810 until an appropriate ratio is achieved. If at step830, the ratio is okay, then the algorithm proceeds to establish astable data set representing the distribution that can be used as aunique key. As mentioned above, in one alternative, the addresses of thememory cells in the various subsets can be combined form a unique dataset, and stored in a protected memory. In the embodiment illustrated inFIG. 8, the process proceeds to apply a biasing operation to memorycells in the third subset to establish a changed distribution ofthresholds having a read margin between the first and second subsets(832), and to record the addresses of the memory cells in the thirdsubset. In this manner, the stable data set can be represented by theaddresses of the third subset, and the data values read from the firstand second subsets sensed using a read voltage within the read marginwhile skipping the memory cells in the third subset (834). Othercombinations of information can be used to establish the stable data setas discussed in various alternatives above. Also, in some embodiments,the biasing step 832 may be omitted.

FIG. 9 is a flowchart 900 for establishing a stable data set using astarting distribution of thresholds in a set of flash memory cells. Theprocess begins with a set of flash memory cells having a startingdistribution of thresholds (901). Next, the process determines theaddresses of a first subset of memory cells having thresholds above thefirst dividing line, a second subset of memory cells having thresholdsbelow a second dividing line, and a third subset of memory cells havingthresholds between the dividing lines (910). A sequence of addresses ofmemory cells in at least one of the subsets is stored in memory separatefrom the set of memory cells (920). The data set can be output as afunction of or equal to the addresses stored in the sequence ofaddresses (930).

FIGS. 10A-10C illustrate one process that can be used for settingthresholds of the charge trapping memory cells in a set of memory cellsto a starting distribution with variant threshold voltages. FIG. 10Aillustrates an initial threshold distribution 816 of a set ofprogrammable memory cells. In this example, the initial thresholddistribution is before a programming operation that induces tunneling ofelectrons or negative charge into the charge storage structure,increasing the threshold voltage of the cells, or an erasing operationthat induces tunneling of electrons out of the charge storage, or ofpositive charge into the charge storage structure, to decrease thethreshold voltage of the cell. In this example, the memory cells in theset can have any initial threshold distribution. In this example, theinitial threshold distribution includes relatively low threshold rangebelow a first verify level.

FIG. 10B illustrates the result of programming all members of the set ofthe programmable memory cells to a threshold range 817 greater than thefirst verify level. This is like a pre-program operation used in flashmemory.

FIG. 10C illustrates the result of erasing all members of the set of theprogrammable memory cells to establish thresholds in the startingdistribution 818, where the starting distribution includes thresholdsbelow a second verify level. The distribution 830 can be utilized as astarting distribution for the processes described above. Othertechniques can be applied as well, including using the range 817 of FIG.10B as a starting distribution.

The technique for establishing a starting distribution described herein,including the techniques described with reference to FIGS. 10A-10C canbe applied to charge trapping memory cells. Examples of charge trappingmemory cells are illustrated in FIGS. 10D, 10E and 10F.

FIG. 10D is a simplified diagram of a planar floating gate memory cell,formed on a substrate 840. A source region 841 and a drain region 842are disposed on opposing sides of the charge trapping structure. Acontrol gate 843 overlies the charge trapping structure, and can be aportion of a word line for example. The charge trapping structureincludes a tunneling layer 844 typically formed of silicon oxide, afloating gate layer 845 typically formed of polysilicon, a blockingdielectric structure including a multilayer ONO structure having anoxide layer 846, a nitride layer 847, and an oxide layer 848. Thephysical function of programming and erasing a floating gate memorycells like that of FIG. 10D induces charge tunneling that changes thecharge trapped in the floating gate 845. The amount of charge trappedvaries according to physical characteristics of each cell, includingprocess variations, temperature variations, voltage variations and thelike. Thus, an operation to establish a starting threshold like thatdescribed above results in threshold voltages in a relatively broaddistribution across a large number of memory cells on a single device.

FIG. 10E is a simplified diagram of a planar dielectric charge trappingmemory cell, formed on the substrate 850. A source region 851 and adrain region 852 are disposed on opposing sides of the charge trappingstructure. A control gate 853 overlies the charge trapping structure,and can be a portion of a word line for example. The charge trappingstructure includes a tunneling layer 856 typically formed of an oxide,or of multiple thin layers of dielectric. Above the tunneling layer 856,a charge trapping layer 857 is disposed, typically comprising siliconnitride or other dielectric materials. Above the charge trapping layer857 is a blocking layer 858, typically formed of another dielectricoxide such as silicon oxide. As with the floating gate memory cell, thephysical function of programming and erasing of memory cells like thatof FIG. 10E induces charge tunneling that changes the charge trapped inthe charge trapping layer 857. The amount of charge trapped variesaccording to the physical characteristics of each cell, includingprocess variations, temperature variations, voltage variations and thelike. Thus, and operation to establish a starting threshold like thatdescribed above results in threshold voltages in a relatively broaddistribution across a large number of memory cells on a single device.

FIG. 10F is a simplified diagram of a 3D vertical NAND flash memorystructure, formed on a substrate 860. A vertical channel structure (e.g.863) is disposed between stacks of word lines (e.g. 867). A chargestorage structure, such as a dielectric charge trapping structure or afloating gate structure, is disposed between the word lines and thevertical channel structure 863. The vertical channel structure 863 iscoupled to a bit line 869. In the substrate, a common source conductoris disposed, establishing a current path for the NAND string between thebit line 869, and the substrate 860 through the vertical channelstructure 863. The flash memory structure shown in 10F can also be usedto establish a relatively broad distribution of threshold voltagesacross a large number of memory cells on the device, using a program orerase operation.

Other types of flash memory cell structures, including other 3D memorytechnologies can be deployed as well for the purposes of the PUFprocedures described herein.

FIG. 11 shows an example flowchart 300 for applying an initializingbiasing operation setting thresholds of the programmable memory cells inthe set to the starting distribution which can be applied to chargestorage memory cells, including memory cells like those discussed above.At Step 310, all members of the set of the programmable memory cells areprogrammed to a threshold range greater than a first verify level. Inone example, all members of the set of the programmable memory cells canbe programmed using a programming algorithm known as incremental steppulsed programming sequence (ISPP), with increasing pulse height andprogram verify steps being executed until desired threshold levels aresatisfied. At Step 320, all members of the set of the programmablememory cells are erased to establish thresholds in the startingdistribution, the starting distribution including thresholds below asecond verify level.

The verify levels used for the program or erase operations according tothis process to establish a starting distribution can be the same asused for the program and erase operations applied to a large-scalememory on the same integrated circuit. Alternatively, the verify levelsused to establish the starting distribution can be adjusted as suits aparticular implementation, so that the starting distribution has desiredcharacteristics for use in creating a data set as described herein.Although an “erase” process, where net positive charge is added to thecharge trapping structure reducing the threshold of the cells, is usedin this example to produce the starting distribution, “program”processes, where net negative charge is added to the charge trappingstructure increasing the threshold of the cells can also be used. Also,as mentioned above, the starting distribution can be the “initial”distribution of thresholds that results from the manufacturing processesor other processes to which the set of memory cells is exposed. The“initial” distribution and distributions resulting from erase or programoperations can all be considered physical unclonable functions.

Also, for non-volatile memory based on programmable resistance memorycells, a “set” process, where net reduction in resistance is causedreducing the threshold voltage for a read current of the cells, can beused to produce the starting distribution. Alternatively, “reset”processes, where net increase in resistance is caused increasing thethreshold voltage for a read current of the cells can also be used.Also, as mentioned above, the starting distribution can be the “initial”distribution of thresholds that results from the manufacturing processesor other processes to which the set of programmable resistance memorycells is exposed. The “initial” distribution and distributions resultingfrom set or reset operations can all be considered physical unclonablefunctions.

FIGS. 12A-12C illustrate another technique for generating a data setusing a PUF function for charge trapping memory cells, of the typedescribed with reference to FIG. 9. In FIG. 12A, a starting distribution1200 is illustrated such as might be produced using a PUF, like an eraseoperation that moves the thresholds of memory cells in a set of memorycells below an upper threshold level using an erase verify function. Thestarting distribution can be characterized as having an upper thresholdlevel and a lower threshold level as indicated in the figure, and memorycells having thresholds within the distribution 1200 can becharacterized for the purposes of this description, as representing thedata value “0” for a standard read operation configured for a memoryarray.

FIG. 12B illustrates a next step in establishing the data set. In thisexample, the memory cells in the set are read using a moving readvoltage level V_(R) starting from one side of the distribution 1200.Assuming that the moving read voltage starts from a lower thresholdlevel, the set of memory cells is read using a read voltage and a countof the number of memory cells having thresholds above and below thatthreshold is determined. This read voltage is moved until the numbers ofmemory cells above and below the read voltage match a desired parameter,such as being about equal, or having a ratio of about one. At thisstage, memory cells having thresholds below the read voltage V_(R)within the sub-distribution 1210 can be characterized as representingthe data value “1”, and the memory cells having thresholds above theread voltage V_(R) within the sub-distribution 1220 can be characterizedas having a data value “0”. For example, the reading operation cancontinue until a ratio of a count of the programmable memory cellshaving threshold levels below the read level to a count of theprogrammable memory cells having threshold levels above the read levelis within a target range of ratios. For example, a ratio within thetarget range of ratios can be substantially equal to 1, when the countof the programmable memory cells having threshold levels below the readlevel corresponds to about 50% of the memory cells in the set. A readlevel at which the ratio is within the target range of ratios, such as40% to 60% for example, can be established as a read voltage V_(R) forreading memory cells in the set of programmable memory cells to generatethe stable data set.

FIG. 12C illustrates a next step in establishing the data set. Accordingto this technique, an upper read voltage V_(R)+ and a lower read voltageV_(R)− are chosen in order to define sub-distributions of the startingthreshold distribution 1200 in which the memory cells strongly store thedata values. Once the read voltage V_(R) is determined as described inconnection to FIG. 12B, first and second dividing lines can beestablished in the distribution by establishing a sensing margin aroundthe read voltage V_(R), so that the sensing margin (the difference 1240between V_(R)− and V_(R)+) is between the first and second dividinglines, and the read voltage V_(R) is within the sensing margin. Forinstance, the first dividing line can be at a threshold levelcorresponding to the read level minus 300 mV for one particular type offlash memory cell, while the second dividing line can be at a thresholdlevel corresponding to the read level plus 300 mV. In another example,the first dividing line can be at a threshold level V_(R)− correspondingto the read level V_(R) minus 30% of the read level, while the seconddividing line can be at a threshold level V_(R)+ corresponding to theread level V_(R) plus 30% of the read level.

The data set can be generated using the memory cells within thesub-distribution 1211 and the sub-distribution 1221 which “strongly”store the data values “1” and “0”. The addresses of such memory cellsare recorded in memory on the integrated circuit, such as in a stableflash memory block, a different type of non-volatile memory, or involatile memory such as SRAM or DRAM for use by the integrated circuitin security protocols such as encryption and authentication, and thelike. A read operation can be executed using the central read voltagevalue V_(R), along with the recorded addresses which provide a strongread margin. In this manner, only memory cells that strongly store datavalues relative to the read voltage are utilized, making the probabilityof an error in reading the data that might occur due to threshold driftsvery low.

FIG. 13 is an example flowchart 1300 for generating a stable data set onan integrated circuit including programmable, charge trapping memorycells in a manner explained with reference to FIGS. 12A-12C. In thisexample, the process begins with a set of flash memory cells having astarting distribution of thresholds (1301), having been established by acommon process which results in variant threshold voltages as discussedabove. In this alternative, a reading operation using a moving readlevel (voltage V_(R)) is executed on memory cells in the set (Step1310). The process determines a ratio of a count of the programmablememory cells having threshold levels below a current read level to acount of the programmable memory cells having threshold levels above thecurrent read level (Step 1320). Then the process determines whether theratio is within a target range of ratios. (Step 1330). For instance, aratio within a target range of ratios can be substantially equal to 1,when the count of the programmable memory cells having threshold levelsbelow the current read level corresponds to about 130% of the memorycells in the set. If the ratio is not within the target range of ratios(Step 1330, No), the read level can be adjusted (Step 1340), for exampleby incrementing the read level, where for a first iteration of Step1310, the read level can start from a threshold level at or below aminimum threshold level of the distribution. The reading operation thenreturns to Step 1310 and continues until the ratio is within the targetrange of ratios (Step 1330, Yes). A read level at which the ratio iswithin the target range of ratios is established as a read voltage V_(R)(FIG. 12B) for reading memory cells in the set of programmable memorycells to generate the stable data set (Step 1350).

If at Step 1330, the ratio is within a target range of ratios, then theprocess proceeds to establish one or both of first and second dividinglines in the distribution based on desired characteristics of the dataset (Step 1340). For instance, the process can establish a sensingmargin around the read voltage V_(R) in the starting distribution (FIG.4B), so that the sensing margin (e.g. 530, FIG. 4C) is between the firstand second dividing lines, and the read voltage V_(R) is within thesensing margin. For instance, the first dividing line can be at athreshold level V_(R)− corresponding to the read level minus 300 mV,while the second dividing line can be at a threshold level V_(R)+corresponding to the read level plus 300 mV. For instance, the firstdividing line can be at a threshold level corresponding to the readlevel minus 30% of the read level, while the second dividing line can beat a threshold level corresponding to the read level plus 30% of theread level.

The process of FIG. 13 includes Step 1360 of determining addresseswithin the set of memory cells having a first subset defined by a firstdividing line, a second subset defined by a second dividing line, and athird subset having thresholds between the first and second dividinglines, and establishing a stable data set representing the distributionthat can be used as a unique key using the determined addresses.

The process of FIG. 13 includes Step 1370 of outputting the data set byreading a sequence of memory cells in the set using a read voltage V_(R)in the sensing margin 530 (FIG. 4). The output data set can be providedto an external system, such as the system controlling execution of thePUF for use as a shared secret in a security protocol. The data set canbe stably stored in the set of memory cells because the sensing marginestablishes a very low likelihood that memory cells initially identifiedas members of the second subset would have their threshold voltagesdrift into the range identified with the first subset.

Using this technique, the data set depends on the number of memory cellswhich are determined to strongly store data values. This number can varyfrom one starting distribution to the next. Thus, in the generation ofthe data set, the sequence of memory cells can be truncated if thenumber of cells is larger than the desired size of the data set, orpadded if the number of cells is smaller than the desired size of thedata set.

FIGS. 14A-14C illustrate yet another example for generating a data setusing a PUF function for charge trapping memory cells of the typedescribed with reference to FIG. 9. In FIG. 14A, a starting distribution1400 is illustrated such as might be produced using a PUF as describedabove. The starting distribution 1400 can be roughly Gaussian havingrelatively symmetric drop-offs as the threshold levels extend away froma central peak. However, the distribution is unlikely to be actuallysymmetric. As mentioned above, this lack of symmetry can result indifferent numbers of memory cells “strongly” storing data in the processdescribed with reference to FIG. 13. According to the technique of FIGS.14A-14C, tighter control over the number of memory cells stronglystoring data can be achieved.

As illustrated in FIG. 14A, a sub-distribution 1410 of memory cells thatcan be characterized as strongly storing the data value “1” can be foundby using first reading operation that applies a moving read level,starting at a threshold at or near the lower boundary of thedistribution 1400, and executing a process for iteratively reading thememory cells in the set of memory cells using a current read voltage,and counting the number of memory cells that have thresholds below thecurrent read level. When the count reaches a specified threshold, thenthe current read level is stored as the first, lower dividing linevoltage V_(R)−.

As illustrated in FIG. 14B, a second sub-distribution 1420 of memorycells that can be characterized as strongly storing the data value “0”can be found by using a second reading operation that applies a movingread level, starting at a threshold at or near the upper boundary of thedistribution 1400, and executing a process for iteratively reading thememory cells in the set of memory cells using a current read voltage,and counting the number of memory cells that have thresholds above thecurrent read level. When the count reaches a specified threshold, thenthe current read level is stored as the second, upper dividing linevoltage V_(R)+.

As illustrated in FIG. 14C, a third sub-distribution 1430 includesmemory cells having thresholds between the first dividing line voltageV_(R)− and the second dividing line voltage V_(R)+. Using addresses ofthe memory cells that fall within the first sub-distribution 1410“strongly” storing the data value “1”, and within the secondsub-distribution 1420 “strongly” storing the data value “0,” a data setcan be generated by reading the memory cells using a read voltage V_(R)between the first and second dividing lines. This read voltage can beproduced by averaging the first dividing line voltage V_(R)− and thesecond dividing line voltage V_(R)+ where the starting distribution canbe characterized as roughly Gaussian. In embodiments where the startingdistribution may be skewed toward higher or lower thresholds, then theread voltage can be produced using a formula that takes into account theskew in the distribution.

FIG. 15 is an example flowchart 1500 for generating a stable data set onan integrated circuit including programmable, charge trapping memorycells in a manner explained with reference to FIGS. 14A-14C. In thisexample, the process begins with a set of flash memory cells having astarting distribution of thresholds (1501), having been established by acommon process which results in variant threshold voltages as discussedabove. In this example, the process includes determining addresseswithin the set of memory cells having a first subset of thresholds belowa first dividing line and a second subset having thresholds above asecond dividing line. The dividing lines are determined in a manner thatprovides a predetermined number of memory cells that strongly store thedata value “0” and a predetermined number of memory cells that stronglystore the data value “1”. The dividing lines can be stored as aparameter in the system controlling execution of the PUF.

In this alternative, a first reading operation using a first moving readlevel is executed on memory cells in the set (Step 1510). The processdetermines a first count of the programmable memory cells havingthreshold levels below the first read level V_(R)− (Step 1512). Then theprocess determines whether the first count matches a predeterminednumber, or falls within a range of numbers (Step 1514).

If the first count is not accepted (Step 1514, No), the first read levelcan be adjusted (Step 1516), for example by incrementing the first readlevel, where for a first iteration of Step 1510, the first read levelcan start from a threshold level at or below a lower threshold level ofthe distribution. The first reading operation then returns to Step 1510and continues until the first count is accepted (Step 1514, Yes).

The process of FIG. 15 includes establishing a first dividing linevoltage V_(R)− in the starting distribution as a first read level atwhich the first count is at or close to a predetermined number (Step1518). Addresses of memory cells having threshold levels below the firstdividing line V_(R)− can be determined and stored for establishing astable data set representing the distribution that can be used as aunique key, at this step or at a later step when a second dividing lineis established.

The process of FIG. 15 includes Step 1520 of a second reading operationusing a second moving read level V_(R)+ on memory cells in the set (Step1520). The process determines a second count of the programmable memorycells having threshold levels above the second read level (Step 1522).Then the process determines whether the second count is acceptable, forexample if it matches a predetermined number, or falls within a range ofnumbers (Step 1524). The number can be accepted in some embodiments ifthe sum of the count from the first reading operation and the count fromthe second reading operation is equal to or within a range of a targetnumber of bits for the data set, or a target number of addresses for thedata set.

If the second count is not accepted (Step 1524, No), the second readlevel can be adjusted (Step 1526), for example by decrementing thesecond read level, where for a first iteration of Step 1520, the secondread level can start from a threshold level at or above an upperthreshold level of the distribution. The second reading operation thenreturns to Step 1520 and continues until the second count is accepted(Step 1524, Yes).

The process of FIG. 15 includes establishing a second dividing line inthe starting distribution as a second read level V_(R)+ at which thesecond count is accepted, and establishing a read voltage V_(R) usingthe first and second dividing lines, such as by averaging according toan equation: V_(R)=(V_(R)−+V_(R)+)/2 (Step 1528).

Although as shown in FIG. 15, iterations including Steps 1510, 1512,1514 and 1516 to establish the first dividing line are executed beforeiterations including Steps 1520, 1522, 1524 and 1526 to establish thesecond dividing line are executed, in other embodiments, iterationsincluding Steps 1520, 1522, 1524 and 1526 to establish the seconddividing line can be executed before iterations including Steps 1510,1512, 1514 and 1516 to establish the first dividing line.

The process of FIG. 15 can continue with a step like Step 1360 of theprocess of FIG. 13, determining addresses within the set of memory cellsof a first subset of memory cells defined by the first dividing linestrongly storing the data value “1” and a second subset defined by thesecond dividing line strongly storing the data value “0”. Addresses ofthe third subset of memory cells that do not strongly store data values,having thresholds between the first and second dividing lines, can alsobe used in establishment of the data set in some embodiments.

The process of FIG. 15 can continue with a step like Step 1340 of theprocess of FIG. 13, outputting the data set by reading a sequence ofmemory cells in the set using a read voltage V_(R) in the sensingmargin. The output data set can be provided to an external system, suchas the system controlling execution of the PUF for use as a sharedsecret in a security protocol. The data set can be stably stored in theset of memory cells because of the sensing margin between V_(R)− andV_(R)+ establishes a very low likelihood that memory cells initiallyidentified as members of the first and second subsets would have theirthreshold voltages drift into the range identified with the othersubset.

In one embodiment, a predetermined length T in the number of cells to beused for a completed data set to be used, by specifying first and secondpredetermined lengths T1 and T0 to the first and second subsets of theset of the programmable memory cells, respectively, where T=T1+T0,indicating a number of bits in the data set or a number of memory cellsin the first and second subsets in the set of the programmable memorycells. For instance, the first and second target ranges of counts caninclude the first and second predetermined lengths T1 and T0, so thatthe first and second dividing lines in the starting distribution can beestablished as first and second read levels at which the first andsecond counts match the first and second predetermined lengths T1 andT0, respectively. When the sum of the numbers do not match a specifiedlength if the data set is so restricted, because the granularity of themoving read operations can be greater than one cell, the excess cell canbe removed from the data set, or the data set can be padded with data toform the corrected length.

FIG. 16 illustrates an integrated circuit 1600 that includes a flashmemory array 1610 comprising a set of PUF memory cells, which can besubjected to a PUF to establish a distribution of thresholds asdiscussed above. The integrated circuit 1600 includes a PUF controller1630 and Security Logic 1640 as mentioned above in connection withFIG. 1. Also, access and bias circuits 1620 are provided that enable useof the flash memory array 1610, including word line drivers, senseamplifiers, bit line drivers, voltage sources and other circuitsperipheral to the flash memory array. The PUF controller 1630 in thisexample is connected to the access and bias circuits 1620 of the flashmemory array 1610 and includes logic and memory resources used to carryout the processes described herein, including for example some or all ofthe processes of FIGS. 6, 8, 9, 11, 13 and 15.

In the illustrated embodiment, a state machine 1633 and an address andparameter store 1632 are included in the PUF controller 1630. The statemachine 1633 can include logic used to generate a data set based on thePUF applied to the set of memory cells in the array 1610. In embodimentsof the technology described herein, the logic can perform the steps offinding the subsets or sub-distributions of memory cells read in thegeneration of the data set, recording the parameters in the store 1632such as thresholds used for the dividing lines discussed above,thresholds used for reading the data values from the identified subsets,and recording in the store 1632 the addresses of the memory cellsidentified for use in generating the data set. The logic can alsoperform the steps of applying the read voltages and the addresses storedin the store 1632 to produce sequences of data values from the flashmemory array 1610.

The state machine can also include logic to cause scanning ofprogrammable memory cells in a set of programmable memory cells, andapply the processes described herein to produce a stable data set basedon a physical unclonable function.

The security logic 1640 can include logic for handling a challenge inputand providing a response output using the data set read from the array1610 or form the store 1632 or from both. The security logic 1640 caninclude encryption and decryption resources using the data set, and caninclude logic to control an authentication protocol using the data set.The response may be a pass/fail signal in some embodiments applied onthe integrated circuit to enable mission function circuits for example.In other embodiments, the response may be applied to circuitry off ofthe integrated circuit 1600 for which the data set is used in a securityprotocol. In some embodiments, the security logic includes a statemachine implemented using dedicated logic, general purpose processorwith suitable programming, a programmable gate array with suitableprogramming or a combination of these types of logic circuits. Also, thesecurity logic 1640 can share the logic used to implement the statemachine 1633 in some implementations.

The store 1632 can be implemented using non-volatile memory, such asflash memory, programmable resistance memory, one-time-programmablememory and the like. Also, the store can be implemented using othertypes of memory, including volatile memory such as SRAM, with backupcopies of the addresses and parameters stored in the array 1610 or inother memory accessible to the integrated circuit.

The state machine 1633 can be implemented using dedicated logic, ageneral purpose processor with suitable programming, a programmable gatearray with suitable programming or a combination of these types of logiccircuits.

Thus, FIG. 16 illustrates an example of an integrated circuit,comprising a set of programmable memory cells on an integrated circuithaving a distribution of thresholds; memory storing addresses of memorycells in the set of programmable memory cells that have thresholds in afirst sub-distribution of the distribution of thresholds; and logic togenerate a data set using the stored addresses.

The distribution can be characterized by having been made using aphysical unclonable function.

In some embodiments, the first sub-distribution is separated from asecond sub-distribution by a sensing margin, and the logic to generatethe data set includes logic to read the memory cells in the set ofprogrammable memory cells in address order to generate data values thatvary according to membership or not in the first sub-distribution.

In some embodiments, the memory stores, in addition, addresses of memorycells in the set of memory cells that have thresholds in a secondsub-distribution of a distribution of thresholds of memory cells in theset; and the logic to generate the data set includes using the storedaddresses for the first sub-distribution and the secondsub-distribution.

In some embodiments, the memory stores, in addition, a first dividingline and a second dividing line different than the first dividing line,for distribution of thresholds; and the memory cells in the firstsub-distribution include a first subset of the set of the memory cellshaving thresholds below the first dividing line, and the memory cells inthe second sub-distribution include a second subset of the set of thememory cells having thresholds above the second dividing line.

In some embodiments, the logic to generate the data set uses theaddresses to select memory cells in one of the first and second subsets;and reads memory cells in the set of programmable memory cells using aread voltage between the first and second dividing lines.

In some embodiments, the programmable memory cells in the set are chargetrapping memory cells, and the thresholds are threshold voltages.

In some embodiments, the integrated circuit can include logic to applybiasing operations using biasing circuits on the integrated circuit thatinduce changes in the charge storage structures of the programmablememory cells in the set to establish the distribution.

In some embodiments, the logic comprises a state machine on theintegrated circuit.

In some embodiments, the integrated circuit includes logic that respondsto a challenge input to generate a response output using the data set.

FIG. 17 illustrates a system including a packaged integrated circuit ormultichip module 180 that includes an input/output interface 181 and anon-volatile memory array 185. The input/output interface 181 provides aport for external communication of data between external devices orcommunication networks and the non-volatile memory array 185. The memoryarray 185 includes a plurality of blocks of memory cells (see, e.g.,FIG. 3 above) with a security key stored in a particular block 187 ofthe plurality of blocks. Security logic 190 is coupled to thenon-volatile memory array 185, which utilizes the security key in aprotocol to enable access to data stored in blocks in the plurality ofblocks. Access control circuits including the access control switch 183are coupled to the array, and include logic to enable read-only accessto the particular block by the security logic for use in the protocol,and to prevent access to the particular block via the port by externaldevices or communication networks. Other combinations of access rulescan be used in various embodiments, allowing the security logic greaterflexibility in the utilization of the particular block.

In this example, the non-volatile memory array 185 comprises flashmemory. The particular block 187 storing the security key can bephysically located anywhere in the array, but as illustrated can belocated physically in a top block having the lowest physical address, oradjacent a boot block having a lowest physical address, for a couple ofexamples.

The non-volatile memory array 185 is coupled to sense amplifiers/buffers184 which provide for flow of data into and out of the flash memoryarray, including the particular block 187 storing the security key. Theaccess control switch 183 is disposed in this example between the senseamplifiers/buffers 184 and the input/output interface 181. The data readfrom the array 185 can be routed on path 182 to the input/outputinterface 181, or can be routed on path 191 to the security logic 190.

In the illustrated embodiment, an address decoder 186 is coupled to theflash memory array 185, along with block lock bits which are used forcontrolling permission to read and write data in corresponding blocks inthe array, including the particular block 187. The block lock bitsutilized for that particular block 187 can comprise a different logicalstructure than that used for the other blocks in the array, and performlogically a different function. For example, the block lock bitsassociated with the particular block 187 that stores the security keycan control logic coupled to the access control switch 183 that preventsdata flow from the particular block 187 through the senseamplifiers/buffers on line 182 to the input/output interface 181, whileallowing the data flow from the particular block 187 on line 191 to thesecurity logic 190, when an address used to access the array correspondsto the address of the particular block 187.

Also in the illustrated embodiment, a flash control state machine 193with a physical unclonable function program controller is coupled to thememory array 185 on line 194, and to the security logic 190 on line 192.The physical unclonable function can perform procedures as describedherein, using memory cells in a particular set 189 of memory cells inthe array 185 for the purposes of producing a data set to be used as thesecurity key. In this example of the apparatus, flash control statemachine 193 provides signals to control the application of biasarrangement supply voltages to carry out the procedures to generate thedata set, and other operations involved in accessing the array 185.Circuitry, which is on the integrated circuit such as bit lines, wordlines, drivers for the same and so on, provides access to the set offlash memory cells used to provide a data set used to produce thesecurity key.

As illustrated, packaged integrated circuit or multichip module 180 canalso include other circuitry 195, such as can be encountered in asystem-on-a-chip system or other combinations of circuitry with memory.

The packaged integrated circuit or multichip module 180 is coupled inthe example shown to an enrollment system 198 by interconnect 199. Theenrollment system 198 can maintain a key database 198A in whichinformation needed to perform the security protocol relying on thesecurity key stored in the particular block 187 can be to maintained. Insome embodiments, the information needed to perform the securityprotocol includes a copy of the security key.

In one example operating method, during manufacture or packaging, thephysical unclonable function can be executed by the flash control statemachine 193, in cooperation with the enrollment system 198 as discussedabove with reference to FIG. 2. The physical unclonable function canutilize the set of memory cells 189 to produce a data set that can beused to form the security key. The data set upon completion of theexecution of the physical unclonable function can then be copied fromthe set of memory cells 189 to the particular block 187 reserved orconfigured for storing the security key. The system can produce one ormany security keys for storage in the particular block 187 reserved forthis purpose. At this stage, the security key can also be copied intothe enrollment system 198 and maintained with the key database 198A.After the execution of the physical unclonable function, and the copyingof the security key into the particular block 187 and any necessaryinformation into the enrollment system, the lock bits associated withthe particular block 187 can be set using a fuse or other type of writeonce memory element, to prevent access to the security key by externalcircuits or communication networks. Also, the particular set 189 ofmemory cells used in the physical unclonable function can be erased orotherwise overwritten to eliminate evidence of the security key that maybe stored in the memory array 185.

FIGS. 18 and 19 illustrate different examples for the configuration ofthe non-volatile memory array for different embodiments. In FIG. 18, theparticular block of memory cells in which the security key is storedcomprises a first sub-block 187A and a second sub-block 187B. In thefirst sub-block 187A, the set of memory cells used by the physicalunclonable function to produce the security are located. Also, thesecurity key can be kept in the set of memory cells used to produce thedata set, or moved to another set of memory cells in the sub-block 187A.The second sub-block 187B maintains a cell map or multiple cell mapsgenerated during execution of the physical unclonable function accordingto a process such as those described with reference to FIGS. 7A-7E, 8,12A-12C and 13, for example.

FIG. 19 is an alternative in which the set of memory cells 189 used bythe physical unclonable function is in the flash memory array 185, andoutside of the particular block 187 used for storage of the securitykey. In this example, the particular block used for storage of thesecurity key includes a first sub-block 187C in which the security keyor multiple security keys are maintained in the memory. The secondsub-block 187B maintains the cell map or multiple cell maps generatedduring the physical unclonable function.

FIG. 20 illustrates a data structure which can be utilized for storageof a security key and cell map in embodiments like those shown FIGS.17-19. The set of memory cells used to produce the security key(security ID in this example) is represented by the security IDgenerator block. This block has a “block” address which identifies thestarting location, and bit addresses 1 through 10 in the figure. In apreferred system, the security ID generator block may have thousands ofbits. Also, associated with each bit address is a data value providing“code information,” which indicates a data value sensed using theprocedure of FIG. 8 for example or FIG. 19 for example. In embodimentsutilizing a mapping table or cell map as shown in FIG. 20 to address thedata set, the data values in some of the cells are not used in the key,and so are considered “don't care” cells. The mapping table identifiesthe addresses of the “don't care” cells and the cells used for thesecurity key. Thus, the mapping table in this example has a startingaddress, and address bits 1 through 10 which correspond to the bitaddresses 1 through 10 of the cells in the security ID generator block.Flags are set in memory cells corresponding to each of the address bits,indicating a valid cell (used in the key) or an invalid cell (not usedin the key) in the security ID generator block. The key data can begenerated by a logical AND of the mapping table and the codeinformation, where the mapping table is used as a mask. As mentionedabove, the security ID generator block is a set of memory cells that canbe located anywhere in the non-volatile memory array, or located in theparticular block used for storage of the security ID. The data in thesecurity ID generator block can then be copied to the particular blockused for storage of the security ID in embodiments in which the set ofmemory cells is located outside of particular block.

FIG. 21 illustrates a high level configuration of a system utilizing aphysical unclonable function for the generation of a security key, andstorage of that key in a non-volatile memory. The system includes a host1720 which is coupled to an integrated circuit or multichip module 1710.The integrated circuit or multichip module 1710 includes a physicalunclonable function circuit 1711, controller 1712, and security logic1713. The controller 1712 is coupled to the physical unclonable functioncircuit 1711 and to a non-volatile memory 1714.

Operation of the system of FIG. 21 can be understood for someembodiments with reference to FIG. 22. Thus, in order to produce a keythat can be used, key data is generated from the physical unclonablefunction circuit 1711 (step 1730). The key is analyzed to determinewhether it meets security specifications, such as having sufficientrandomness (1731). If the key meets the specifications, then it storedinto the non-volatile memory 1714 via the controller 1712 (step 1732).If it does not meet the specifications, then the process loops to step1730 to retry the PUF to produce a key. The physical unclonable functioncan use a set of non-volatile memory cells as described above to producea security key of any length and to retry the key production processbased on the PUF. As illustrated, the PUF circuit 1711 and controller1712 will cooperate to generate another key, looping back to step 1730until a satisfactory key is produced. Otherwise, key generation iscompleted, the key or keys are stored and ready to be utilized by thesecurity logic. To use the key, the process includes getting the keydata from the non-volatile memory (1733), and executing the securityfunction in a protocol involving the host 1720 and the key data for oneor more keys in the non-volatile memory (1734). The host 1720 can beprovided data needed to execute a security protocol that relies on thesecurity key by an enrollment system, or can be the system used duringset up of the key. The security function can be configured incooperation with an enrollment system or communications server, toutilize a plurality of security keys. In some embodiments, the securitykeys generated and stored are utilized only one time, or a limitednumber of times to maintain high security and immunity from snooping.Also in some embodiments, a single large key can be utilized in a mannerthat relies upon subsets of the large key for each communicationsession. Other security protocols can be implemented as needed for aparticular environment of use. In the process of FIG. 22, duringutilization of the key by the security process, a key update protocolcan be executed which signals that the key should be updated. This caninclude replacing the key after a period of time, or after a fixednumber of uses. Also, the key can be replaced if a number of log inattempts are used that fail or other events are detected, suggesting anattempt to guess the key. Thus, the process of FIG. 22 includes the stepof determining whether to update the key (1735). If the key needs to beupdated, then the process loops to step 1730, and the PUF is executed toupdate the key or keys. If the key does not need to be updated at 1735,then the process loops to continue use of the key in support ofexecution of the security function.

When using the PUF to create new keys in the cycling represented bysteps 1731 and 1735 of FIG. 22 for example, the parameters used in thePUF can be shifted in some embodiments to increase the chance ofproducing keys that are substantially different in each cycle. Of coursein some embodiments using flash memory cells, the same PUF parametersapplied to the same cells can produce sufficiently different keys. Inother examples, bias voltages used to produce the initial distribution,can be changed for each new PUF cycle. Also, the number of pulsesapplied in an Incremental Step Pulse Programming ISPP algorithm can bechanged for producing distributions as part of the PUF. In yet otherexamples, the memory cells utilized in producing the pulse can bechanged from a set in one area of the array to a different set inanother area of the array.

The high-level function can be considered in two parts in someembodiments such as shown in FIGS. 23 and 24. FIG. 23 illustrates afunction that can be executed during manufacture, or before shipment tothe customer, or otherwise before a security key has been used by thesystem. In FIG. 23, the procedure begins with a power-on event (1750).The physical unclonable function is executed and the key data includingone or more keys is retrieved and provided to an enrollment system orother external system that will need the key data (1751). The key datais stored in a non-volatile memory as discussed above (1752). The keydata is protected after it has been stored in the non-volatile memory,from access by external communication networks or devices (1753). In thefield, the procedure flows generally as shown in FIG. 24, beginning witha power-on event (1760). The procedure includes getting the protectedkey data from the non-volatile memory (1761), and executing a securityfunction including a communication protocol, such as achallenge-response exchange, with an external device using the key(1762).

As illustrated in FIG. 25, the physical unclonable function can usephysical circuits 1770 such as SRAM circuits, programmable resistancememory cell circuits RRAM, metal-based circuits, delay based circuits,oscillator based circuits and the like. Typically, the circuits used inphysical unclonable functions have relatively low stability, requiringspecial logic or error correction in order to reliably use the keys. Thenon-volatile memory 1771 used for storage of the security key cancomprise non-volatile memory that is highly stable, such as flashmemory, programmable resistance memory RRAM, phase change memory PCRAM,one time programmable memory and the like. In other embodiments, thecircuits 1775 used by the physical unclonable function can haverelatively high stability. However, the keys can also be stored innon-volatile memory 1776 which also has high stability, but can alsoprovide better access control and other functions that may not betypically associated with the physical unclonable function circuits1775.

In some embodiments, as represented by FIG. 27, a random numbergenerator 1780 can be utilized to produce a security key, which can thenbe stored in non-volatile memory 1781 and used in systems such as thosedescribed herein.

In some embodiments, as represented by FIG. 28, a physical unclonablefunction circuit 1785 can produce security information at a first levelhaving for example 1024 bits. This security information at the firstlevel can be provided to a logic circuit 1786 which transforms the firstlevel data, using for example a hash function, into second levelinformation having for example 128 bits, or other combination of bitsthat is generated as a function of the security information at the firstlevel. The second level information can then be stored into thenon-volatile memory 1787.

In one aspect of the technology, the computer program controllingexecution of processes like those shown in FIGS. 6, 8, 9, 11 13 and 15and other procedures described herein, can be stored as instructions ona computer readable memory or more than one memory, where the memorycomprises a non-transitory computer readable data storage medium. Usingthe computer readable memory a PUF machine (e.g. 410, FIG. 2) can causescanning of programmable memory cells in a set of programmable memorycells, and apply the processes described herein to produce a stable dataset based on a physical unclonable function.

Also, as mentioned above, an integrated circuit including the set ofprogrammable memory cells can include a state machine or other logicresources configured to execute these processes. In yet otheralternatives, a combination of a computer program executed by a PUFmachine, and logic implemented on the integrated circuit can beutilized.

In embodiments described herein, a set of memory cells having a startingdistribution of threshold voltages is used to establish a stable dataset. This set of memory cells can be part of a large scale memory array,such as shown in FIG. 3, FIG. 16 and FIG. 17. Alternatively, the set ofmemory cells can be a specially provided set of memory cells. In anembodiment in which the mission functions of the integrated circuitinclude a memory array, the set of memory cells used for this purposecan have the same structure as the memory cells in the array, or canhave different structures. Also, the set of memory cells used can bedisposed in any pattern on the integrated circuit, including a compactarray pattern or a distributed pattern.

In embodiments, a set of memory cells used for establishing a startingdistribution can be reused many times to produce multiple stable datasets having variant contents. Thus, logic can be provided in a systemdeploying such embodiments, to utilize the PUF process on memory cellson one integrated circuit to generate unique data sets that can beshared among other devices in communication with the one integratedcircuit.

As mentioned above, the examples described herein are based on usingcharge trapping memory cells, such as flash memory. The technology insome embodiments, including in embodiments configured as shown in FIG.3, FIG. 16 and FIG. 17, is extendable to other programmable memory celltechnology, including programmable resistance cells based on metaloxides, programmable resistance cells based on phase change materials,magneto-resistive memory and other kinds of memory cell technologiescharacterized by an ability to be used to establish the startingdistribution in which threshold voltages or threshold resistances varyrandomly relative to the addresses of the memory cells as a result ofbeing exposed to a common process.

The data set generated as described herein can have content unique tothe particular integrated circuit. The data set can be used to form aresponse to a challenge, such as in the example of security protocols.The data set can be used as a key in an encryption protocol. The dataset can be used as a unique identifier. The data set can be used as arandom key.

Various aspects of the technology described herein include the followingembodiments.

A method for generating a data set on an integrated circuit including aset of programmable memory cells is described in one embodiment. Themethod can comprise exposing the set of programmable memory cells havingaddresses on the integrated circuit to a common process inducing variantthresholds in the programmable memory cells in the set within a startingdistribution of thresholds. The method also can comprise (1) finding afirst subset of the set of programmable memory cells having thresholdsin a first part of the starting distribution, and a second subset of theset of programmable memory cells having thresholds in a second part ofthe starting distribution; and (2) using the addresses of at least oneof the first and second subsets to generate the data set.

The common process can comprise etching or deposition steps duringmanufacturing which induce charge trapping in charge storage structuresof the programmable memory cells in the set. The common process also cancomprise biasing operations using biasing circuits on the integratedcircuit that induce changes in charge storage structures of theprogrammable memory cells in the set.

A method of manufacturing an integrated circuit is described in oneembodiment. The method can comprise forming a plurality of programmablememory cells on the integrated circuit; connecting the integratedcircuit to a system configured to exchanges signals with the integratedcircuit; and using the system to generate a data set in a set ofprogrammable memory cells in the plurality of programmable memory cellshaving a starting distribution of thresholds by (1) finding a firstsubset of the set of programmable memory cells having thresholds in afirst part of the starting distribution, and a second subset of the setof programmable memory cells having thresholds in a second part of thestarting distribution; and (2) using addresses of at least one of thefirst and second subsets to generate the data set.

An apparatus is described in one embodiment. The apparatus can comprisea set of programmable memory cells on an integrated circuit; logic togenerate a data set using the set of programmable memory cells, whereinthe set of programmable memory cells has a starting distribution ofthresholds, by (1) finding a first subset of the set of programmablememory cells having thresholds in a first part of the startingdistribution, and a second subset of the set of programmable memorycells having thresholds in a second part of the starting distribution;and (2) using addresses of at least one of the first and second subsetsto generate the data set.

A product is described in one embodiment. The product can comprise acomputer readable non-transitory data storage medium storing computerinstructions for a process to generate a data set on an integratedcircuit including a set of programmable memory cells, executable by asystem configured to connect to the integrated circuit. The processdescribed can comprise (1) finding a first subset of the set of theprogrammable memory cells having thresholds in a first part of thestarting distribution, and a second subset of the set of theprogrammable memory cells having thresholds in a second part of thestarting distribution; and (2) using addresses of at least one of thefirst and second subsets to generate the data set.

The finding step described in the embodiments can include determining adividing line between the first part of the starting distribution andthe second part of the starting distribution, so that a ratio of a countof the programmable memory cells in the set having thresholds below thedividing line to a count of the programmable memory cells in the sethaving thresholds above the dividing line is within a target range ofratios.

The using addresses step described in the embodiments can includeselecting the programmable memory cells using the addresses of theprogrammable memory cells in said at least one of the first and secondsubsets, applying a biasing operation to the selected programmablememory cells to establish a changed distribution of thresholds for theset of programmable memory cells, the changed distribution having asensing margin between the first and second subsets; and reading theprogrammable memory cells in the set using a read voltage in saidsensing margin to generate the data set. The using addresses step alsocan include combining the addresses of memory cells in said at least oneof the first and second subsets as a function of membership in said atleast one of the first and second subsets, and using the combinedaddresses as the data set.

A method for generating a data set on an integrated circuit is describedin one embodiment. The integrated circuit includes a set of programmablememory cells, and the programmable memory cells have thresholds in astarting distribution. The method comprises finding a first subset ofthe set of the programmable memory cells having thresholds in a firstpart of the starting distribution, and a second subset of the set of theprogrammable memory cells having thresholds in a second part of thestarting distribution. The method can comprise applying a biasingoperation to establish a changed distribution of the thresholds for theprogrammable memory cells in the set, the changed distribution having asensing margin between the first and second subsets; and providing thedata set using the changed distribution.

A method for generating a data set on an integrated circuit is describedin one embodiment. The integrated circuit includes a set of programmablememory cells, and the programmable memory cells have thresholds in astarting distribution. The method comprises finding a first subset ofthe set of the programmable memory cells having thresholds in a firstpart of the starting distribution, and a second subset of the set of theprogrammable memory cells having thresholds in a second part of thestarting distribution. The method can comprise combining addresses ofthe programmable memory cells in at least one of the first and secondsubsets; and providing the data set using combined addresses.

An apparatus is described in one embodiment. The apparatus can includesa set of charge trapping memory cells; and circuitry having access tothe set of charge trapping memory cells to provide a data set using theset of charge trapping memory cells, the data set being a function ofvariant threshold voltages of different members of the set of chargetrapping memory cells as a result of a common process that inducescharge trapping in charge storage structures in the charge trappingmemory cells in the set. The set of charge trapping memory cells asdescribed has an order and the variant threshold voltages have astarting distribution, and the data set is a function of positions inthe order of a subset of the set of charge trapping memory cells havingthreshold voltages in a part of the starting distribution

A method for generating a data set on an integrated circuit including aset of programmable memory cells is described in one embodiment. Themethod comprises exposing the set of programmable memory cells havingaddresses on the integrated circuit to a common process inducing variantthresholds in members of the set within a starting distribution ofthresholds. The method also comprises (1) finding a first dividing lineand a second dividing line different than the first dividing line, inthe starting distribution; (2) identifying a first subset of the set ofprogrammable memory cells having thresholds below the first dividingline in a first part of the starting distribution, and a second subsetof the set of programmable memory cells having thresholds above thesecond dividing line in a second part of the starting distribution; and(3) generating the data set using addresses of at least one of the firstand second subsets.

A method of manufacturing an integrated circuit is described in oneembodiment. The method can comprise forming a plurality of programmablememory cells on the integrated circuit; connecting the integratedcircuit to a system configured to exchange signals with the integratedcircuit; and using the system to generate a data set in a set ofprogrammable memory cells in the plurality of programmable memory cellshaving a starting distribution of thresholds, by (1) finding a firstdividing line and a second dividing line different than the firstdividing line, in the starting distribution; (2) identifying a firstsubset of the set of the programmable memory cells having thresholdsbelow the first dividing line in a first part of the startingdistribution, and a second subset of the set of the programmable memorycells having thresholds above the second dividing line in a second partof the starting distribution; and (3) generating the data set usingaddresses of at least one of the first and second subsets.

An apparatus is described in one embodiment. The apparatus comprises aset of programmable memory cells on an integrated circuit; and logic togenerate a data set using the set of programmable memory cells, whereinthe set of memory cells has a starting distribution of thresholds, by:(1) finding a first dividing line and a second dividing line differentthan the first dividing line, in the starting distribution; (2)identifying a first subset of the set of the programmable memory cellshaving thresholds below the first dividing line in a first part of thestarting distribution, and a second subset of the set of theprogrammable memory cells having thresholds above the second dividingline in a second part of the starting distribution; and (3) generatingthe data set using addresses of at least one of the first and secondsubsets.

A product is described in one embodiment. The product comprises acomputer readable non-transitory data storage medium storing computerinstructions for a process to generate a data set on an integratedcircuit including programmable memory cells, executable by a systemconfigured to connect to an integrated circuit. The process comprises(1) finding a first dividing line and a second dividing line differentthan the first dividing line, in the starting distribution; (2)identifying a first subset of the set of the programmable memory cellshaving thresholds below the first dividing line in a first part of thestarting distribution, and a second subset of the set of theprogrammable memory cells having thresholds above the second dividingline in a second part of the starting distribution; and (3) generatingthe data set using addresses of at least one of the first and secondsubsets.

The step of finding the first and second dividing lines described in theembodiments can include determining a threshold voltage in the startingdistribution at which a ratio of a count of memory cells havingthresholds below the threshold voltage to a count of memory cells havingthresholds above the threshold voltage is within a target range ofratios, and setting the first dividing line by subtracting a firstconstant from the threshold voltage, and setting the second dividingline by adding a second constant to the threshold voltage. The findingstep also can include iteratively reading data values in the set ofprogrammable memory cells using a moving first read voltage, andcounting memory cells in the set having thresholds below the first readvoltage, and setting the first dividing line using the first readvoltage at which the count is within a first target range of counts. Thefinding step also can include iteratively reading data values in the setof programmable memory cells using a moving second read voltage, andcounting memory cells in the set having thresholds above the second readvoltage, and setting the second dividing line using the second readvoltage at which the count is within a second target range of counts.

The step of generating the data set described in the embodiments caninclude using the addresses to select the programmable memory cells inone of the first and second subsets; and reading the programmable memorycells in the set of programmable memory cells using a read voltagebetween the first and second dividing lines. The generating step alsocan include combining the addresses of the programmable memory cells insaid at least one of the first and second subsets as a function ofmembership in said at least one of the first and second subsets

The common process described in the embodiments can comprise etching ordeposition steps during manufacturing which induce charge trapping incharge storage structures of the programmable memory cells in the set.The common process also can comprise biasing operations using biasingcircuits on the integrated circuit that induce changes in charge storagestructures of the programmable memory cells in the set.

A method for generating a data set on an integrated circuit includingprogrammable memory cells is described in one embodiment. The methodcomprises storing addresses of memory cells in a set of memory cellsthat have thresholds in a first sub-distribution of a distribution ofthresholds of memory cells in the set; and generating the data set usingthe stored addresses.

An integrated circuit is described in one embodiment. The integratedcircuit comprises a set of programmable memory cells on an integratedcircuit having a distribution of thresholds; memory storing addresses ofmemory cells in the set of programmable memory cells that havethresholds in a first sub-distribution of the distribution ofthresholds; and logic to generate a data set using the stored addresses.

The distribution is characterized by having been made using a physicalunclonable function. The first sub-distribution is separated from asecond sub-distribution by a sensing margin, and the logic is configuredto generate the data set to read the memory cells in the set ofprogrammable memory cells in address order to generate data values thatvary according to membership or not in the first sub-distribution. Thememory stores, in addition, addresses of memory cells in the set ofmemory cells that have thresholds in a second sub-distribution of adistribution of thresholds of memory cells in the set; and the logic isconfigured to generate the data set includes using the stored addressesfor the first sub-distribution and the second sub-distribution. Thememory stores, in addition, a first dividing line and a second dividingline different than the first dividing line, for distribution ofthresholds, wherein the memory cells in the first sub-distributioninclude a first subset of the set of the memory cells having thresholdsbelow the first dividing line, and the memory cells in the secondsub-distribution include a second subset of the set of the memory cellshaving thresholds above the second dividing line.

The logic as described in the embodiments is configured to generate thedata set using the addresses to select memory cells in one of the firstand second subsets; and reads memory cells in the set of programmablememory cells using a read voltage between the first and second dividinglines.

The logic as described can be configured to apply biasing operationsusing biasing circuits on the integrated circuit that induce changes incharge storage structures of the programmable memory cells in the set toestablish the distribution, and respond to a challenge input to generatea response output using the data set. The logic can comprise a statemachine on the integrated circuit.

In the embodiments described herein, the programmable memory cells inthe set are charge trapping memory cells, and the thresholds arethreshold voltages.

A memory circuit is described in one embodiment. The memory circuitcomprises (1) a non-volatile memory array including a plurality ofblocks of memory cells, and including a security key stored in aparticular block in the plurality of blocks; (2) a port for externalcommunication of data from the array; (3) security logic coupled to thememory array, which utilizes the security key in a protocol to enableaccess to data stored in blocks in the plurality of blocks; and (4)access control circuits coupled to the array which include logic toenable read-only access to the particular block by the security logicfor use in the protocol, and to prevent access to the particular blockvia the port.

A device comprising a packaged integrated circuit or multichip module isdescribed in one embodiment. The device comprises (1) a non-volatilememory array including a plurality of blocks of memory cells, andincluding a security key stored in a particular block in the pluralityof blocks; (2) a port for external communication of data from the array;(3) security logic coupled to the memory array, which utilizes thesecurity key in a protocol to enable access to data stored in blocks inthe plurality of blocks; and (4) access control circuits coupled to thearray which include logic to enable read-only access to the particularblock by the security logic for use in the protocol, and to preventaccess to the particular block via the port.

A method for operating a circuit including a non-volatile memory arrayis described in one embodiment. The method comprises (1) storing asecurity key in a particular block in a plurality of blocks of thenon-volatile memory array; (2) using a port by external devices orcommunication networks for access data from the array; (3) utilizing, asecurity logic circuit coupled to the non-volatile memory array, thesecurity key stored in the particular block in a protocol to enableaccess to data stored in blocks in the plurality of blocks; (4) enablingread-only access to the particular block by the security logic for usein the protocol, and preventing access to the particular block via theport.

The protocol described herein includes a challenge/response protocolincluding exchange of data via the port.

The access control circuits described herein have a first state in whichaccess to the particular block via the port to write the security key isenabled, a second state in which access to the particular block isdisabled for read or write via the port, and access to the particularblock is enabled for read by the security logic. The access controlcircuits described include block lock bits, which enable and disableaccess to corresponding blocks in the plurality of blocks.

In the embodiments, logic is included in the packaged integrated circuitor multichip module. The logic can store the security key produced usingthe set of memory cells into the particular block, and can execute afunction using a set of memory cells in the memory array to produce thesecurity key. The set of memory cells is in the particular block.

The security key described comprises data values in a subset of the setof memory cells, and an address map identifying members of the subsetfor use by the security logic.

The memory array, the port, the security logic and the access controlcircuits can be disposed on a single integrated circuit.

While the present invention is disclosed by reference to the preferredembodiments and examples detailed above, it is to be understood thatthese examples are intended in an illustrative rather than in a limitingsense. It is contemplated that modifications and combinations willreadily occur to those skilled in the art, which modifications andcombinations will be within the spirit of the invention and the scope ofthe following claims.

What is claimed is:
 1. A method for generating a data set on anintegrated circuit including a set of programmable memory cells,comprising: exposing the set of programmable memory cells havingaddresses on the integrated circuit to a process inducing variantthresholds in members of the set within a starting distribution ofthresholds; finding a first dividing line and a second dividing linedifferent than the first dividing line, in the starting distribution bydetermining a threshold voltage in the starting distribution at which aratio of a count of memory cells in the set of programmable memorycells, having thresholds below the threshold voltage, to a count ofmemory cells in the set of programmable memory cells having thresholdsabove the threshold voltage, is within a target range of ratios, andsetting the first dividing line by subtracting a first constant from thethreshold voltage, and setting the second dividing line by adding asecond constant to the threshold voltage; identifying a first subset ofthe set of programmable memory cells having thresholds below the firstdividing line in a first part of the starting distribution, and a secondsubset of the set of programmable memory cells having thresholds abovethe second dividing line in a second part of the starting distribution;applying a biasing operation to change threshold voltages ofprogrammable memory cells having thresholds between the first and seconddividing lines to establish a changed distribution of thresholds for theset of programmable memory cells, the changed distribution having asensing margin between the first and second subsets; and generating thedata set using addresses of at least one of the first and secondsubsets.
 2. The method of claim 1, wherein said generating the data setincludes using the addresses to select memory cells in said at least oneof the first and second subsets; and reading the programmable memorycells in the set using a read voltage between the first and seconddividing lines.
 3. The method of claim 1, wherein said generating thedata set includes combining the addresses of memory cells in said atleast one of the first and second subsets as a function of membership insaid at least one of the first and second subsets.
 4. The method ofclaim 3, including storing the combined addresses in a memory on theintegrated circuit.
 5. The method of claim 1, wherein the programmablememory cells in the set are charge trapping memory cells, and thethresholds are threshold voltages.
 6. The method of claim 1, wherein theprocess inducing variant thresholds comprises etching or depositionsteps during manufacturing which induce charge trapping in chargestorage structures of the programmable memory cells in the set.
 7. Themethod of claim 1, wherein the process inducing variant thresholdscomprises biasing operations using biasing circuits on the integratedcircuit that induce charges in charge storage structures of theprogrammable memory cells in the set.
 8. The method of claim 1, whereinsaid determining includes: iteratively reading data values in the set ofprogrammable memory cells using a moving first read voltage, andcounting memory cells in the set having thresholds below the first readvoltage, and setting the first dividing line using the first readvoltage at which the count is within a first target range of counts. 9.The method of claim 8, wherein said determining includes: iterativelyreading data values in the set of programmable memory cells using amoving second read voltage, and counting memory cells in the set havingthresholds above the second read voltage; and setting the seconddividing line using the second read voltage at which the count is withina second target range of counts.
 10. A method of manufacturing anintegrated circuit, comprising: forming a plurality of programmablememory cells on the integrated circuit; connecting the integratedcircuit to a system configured to exchange signals with the integratedcircuit; and using the system to generate a data set in a set ofprogrammable memory cells in the plurality of programmable memory cellshaving a starting distribution of thresholds, by: finding a firstdividing line and a second dividing line different than the firstdividing line, in the starting distribution by determining a thresholdvoltage in the starting distribution at which a ratio of a count ofmemory cells in the set of programmable memory cells, having thresholdsbelow the threshold voltage, to a count of the memory cells in the setof programmable memory cells having thresholds above the thresholdvoltage, is within a target range of ratios, and setting the firstdividing line by subtracting a first constant from the thresholdvoltage, and setting the second dividing line by adding a secondconstant to the threshold voltage; identifying a first subset of the setof the programmable memory cells having thresholds below the firstdividing line in a first part of the starting distribution, and a secondsubset of the set of the programmable memory cells having thresholdsabove the second dividing line in a second part of the startingdistribution; applying a biasing operation to change threshold voltagesof programmable memory cells having thresholds between the first andsecond dividing lines to establish a changed distribution of thresholdsfor the set of programmable memory cells, the changed distributionhaving a sensing margin between the first and second subsets; andgenerating the data set using addresses of at least one of the first andsecond subsets.
 11. The method of claim 10, wherein said generating thedata set includes: using the addresses to select memory cells in said atleast one of the first and second subsets; and reading memory cells inthe set of programmable memory cells using a read voltage between thefirst and second dividing lines.
 12. The method of claim 10, whereinsaid generating the data set includes combining the addresses of memorycells in said at least one of the first and second subsets as a functionof membership in said at least one of the first and second subsets. 13.The method of claim 12, including storing the combined addresses in amemory on the integrated circuit.
 14. The method of claim 10, whereinthe programmable memory cells in the set are charge trapping memorycells, and the thresholds are threshold voltages.
 15. The method ofclaim 10, further comprising exposing the plurality of programmablememory cells to a process inducing variant thresholds in members of theset within the starting distribution of thresholds, wherein the commonprocess comprises etching or deposition steps during manufacturing whichinduce charge trapping in charge storage structures of the programmablememory cells in the set.
 16. The method of claim 10, further comprisingexposing the plurality of programmable memory cells to a processinducing variant thresholds in members of the set within the startingdistribution of thresholds, wherein the common process comprises biasingoperations using biasing circuits on the integrated circuit that inducecharges in charge storage structures of the programmable memory cells inthe set.
 17. The method of claim 10, wherein said determining includes:iteratively reading data values in the set of programmable memory cellsusing a moving first read voltage, and counting memory cells in the sethaving thresholds below the first read voltage; and setting the firstdividing line using the first read voltage at which the count is withina first target range of counts.
 18. The method of claim 17, wherein saiddetermining includes: iteratively reading data values in the set ofprogrammable memory cells using a moving second read voltage, andcounting memory cells in the set having thresholds above the second readvoltage; and setting the second dividing line using the second readvoltage at which the count is within a second target range of counts.19. An apparatus, comprising: a set of programmable memory cells on anintegrated circuit; and logic to generate a data set using the set ofprogrammable memory cells, wherein the set of memory cells has astarting distribution of thresholds, by: finding a first dividing lineand a second dividing line different than the first dividing line, inthe starting distribution by determining a threshold voltage in thestarting distribution at which a ratio of a count of memory cells in theset of programmable memory cells, having thresholds below the thresholdvoltage, to a count of the memory cells in the set of programmablememory cells having thresholds above the threshold voltage, is within atarget range of ratios, and setting the first dividing line bysubtracting a first constant from the threshold voltage, and setting thesecond dividing line by adding a second constant to the thresholdvoltage; identifying a first subset of the set of the programmablememory cells having thresholds below the first dividing line in a firstpart of the starting distribution, and a second subset of the set of theprogrammable memory cells having thresholds above the second dividingline in a second part of the starting distribution; applying a biasingoperation to change threshold voltages of programmable memory cellshaving thresholds between the first and second dividing lines toestablish a changed distribution of thresholds for the set ofprogrammable memory cells, the changed distribution having a sensingmargin between the first and second subsets; and generating the data setusing addresses of at least one of the first and second subsets.
 20. Theapparatus of claim 19, wherein said logic includes using the addressesto select memory cells in said at least one of the first and secondsubsets; and reading memory cells in the set of programmable memorycells using a read voltage between the first and second dividing lines.21. The apparatus of claim 19, wherein said logic includes combining theaddresses of memory cells in said at least one of the first and secondsubsets as a function of membership in said at least one of the firstand second subsets.
 22. The apparatus of claim 21, including logic tostore the combined addresses in a memory on the integrated circuit. 23.The apparatus of claim 19, wherein the programmable memory cells in theset are charge trapping memory cells, and the thresholds are thresholdvoltages.
 24. The apparatus of claim 19, wherein a process is applied tothe set of programmable memory cells to establish the startingdistribution, the process comprising etching or deposition steps duringmanufacturing which induce charge trapping in charge storage structuresof the programmable memory cells in the set.
 25. The apparatus of claim19, wherein a process is applied to the set of programmable memory cellsto establish the starting distribution, the process comprising biasingoperations using biasing circuits on the integrated circuit that inducecharges in charge storage structures of the programmable memory cells inthe set.
 26. The apparatus of claim 19, wherein said determiningincludes: iteratively reading data values in the set of programmablememory cells using a moving first read voltage, and counting memorycells in the set having thresholds below the first read voltage; andsetting the first dividing line using the first read voltage at whichthe count is within a first target range of counts.
 27. The apparatus ofclaim 26, wherein said determining includes: iteratively reading datavalues in the set of programmable memory cells using a moving secondread voltage, and counting memory cells in the set having thresholdsabove the second read voltage; and setting the second dividing lineusing the second read voltage at which the count is within a secondtarget range of counts.
 28. The apparatus of claim 19, wherein the logiccomprises a state machine on the integrated circuit.
 29. The apparatusof claim 19, wherein the logic comprises a computer program on a systemconnected to the integrated circuit.
 30. A product comprising: acomputer readable non-transitory data storage medium storing computerinstructions for a process to generate a data set on an integratedcircuit including programmable memory cells, executable by a systemconfigured to connect to an integrated circuit, the process comprising:finding a first dividing line and a second dividing line different thanthe first dividing line, in a starting distribution by determining athreshold voltage in the starting distribution at which a ratio of acount of memory cells, having thresholds below the threshold voltage, toa count of the memory cells having thresholds above the thresholdvoltage, is within a target range of ratios, and setting the firstdividing line by subtracting a first constant from the threshold voltageand setting the second dividing line by adding a second constant to thethreshold voltage; identifying a first subset of the set of theprogrammable memory cells having thresholds below the first dividingline in a first part of the starting distribution, and a second subsetof the set of the programmable memory cells having thresholds above thesecond dividing line in a second part of the starting distribution;causing the integrated circuit to apply a biasing operation to changethreshold voltages of programmable memory cells having thresholdsbetween the first and second dividing lines to establish a changeddistribution of thresholds for the set of programmable memory cells, thechanged distribution having a sensing margin between the first andsecond subsets; and generating the data set using addresses of at leastone of the first and second subsets.
 31. The product of claim 30,wherein said generating the data set includes: using the addresses toselect memory cells in said at least one of the first and secondsubsets; and setting a read voltage between the first and seconddividing lines for use in reading memory cells in the set ofprogrammable memory cells.
 32. The product of claim 30, wherein saidgenerating the data set includes combining the addresses of memory cellsin said at least one of the first and second subsets as a function ofmembership in said at least one of the first and second subsets.
 33. Theproduct of claim 30, wherein said determining includes: iterativelyreading data values in the set of programmable memory cells using amoving first read voltage, and counting memory cells in the set havingthresholds below the first read voltage; and setting the first dividingline using the first read voltage at which the count is within a firsttarget range of counts.
 34. The product of claim 33, wherein saiddetermining includes: iteratively reading the data values in the set ofprogrammable memory cells using a moving second read voltage, andcounting memory cells in the set having thresholds above the second readvoltage; and setting the second dividing line using the second readvoltage at which the count is within a second target range of counts.